Part 3

The supratemporals were observed on only one specimen (K. U. no. 788), (fig. 3). Sutures were difficult to distinguish but the medial margin is presumed to curve to fit and to articulate with the lateral margins of the intertemporals. Lateral margins are smoothly curved but the anterior and posterior margins were broken off. There appears to be no ornamentation on this bone. The supratemporals are much more elongated and curving than those in R. elegans.

The cheek region is nearly complete in one specimen (K. U. no. 788), and scattered parts occur in a few others (see fig. 3). The lacrimojugal of no. 788 is elongate, with both ends curving dorsally. It differs from the lacrimojugal in R. elegans, in which the anterior end extends anteriorly and is not curved dorsally. The posterior and anterior margins are not preserved; the greatest height appears to be posterior. Pores of the suborbital portion of the infraorbital sensory canal are seen on the dorsal surface of the bone. In R. elegans the pores are on the lateral surface. A section of the lacrimojugal on specimen no. 11425, broken at both ends, shows a thin layer of bone perforated by the pores and covering a groove for the canal within the dorsal margin of the bone. Both specimens are unornamented.

A nearly complete postorbital (fig. 3) on specimen no. 788 is nearly triangular, with the apex ventral. The concave anterior margin bears pores of the postorbital part of the infraorbital line. Ornamentation consists of widely spaced, coarse tubercles.

Part of one squamosal is preserved. It is somewhat triangular and its apex is ventral. This bone is associated with the postorbital, subopercular and lacrimojugal on no. 788. The preopercular sensory line passes down the curving ventral margin of this bone, and extends ventrally onto a narrow projection. A low ridge, nearly vertical, passes dorsally from about mid-point of the canal to the dorsal portion. The anterior margin is nearly straight, the ventral margin is concave, and the dorsal margin is convex dorsally but may be incomplete. Perhaps the squamosal and preopercular are fused. The surface appears smooth; the view may be of the medial side. The squamosal of R. elegans is nearly triangular and notably different from that of Synaptotylus newelli.

The subopercular (fig. 3) shows closely spaced tubercles on the lateral surface. The bone is an elongated, irregular triangle with the apex pointing anterodorsally. The margins are incomplete, except for the concave, curving anterior margin.

Numerous operculars (fig. 3) occur in the suite of specimens, both isolated and nearly in place. Each is subtriangular; the apex of the triangle is ventral. A slight convexity projects from the anterodorsal border. The posterior margin is broadly but shallowly indented. Otherwise the margins are smooth. Maximum height ranges from 8.0 to 11.0 mm., and maximum width from 8.0 to 13.0 mm. Ornamentation varies from a few widely spaced, randomly oriented tubercles to closely spaced tubercles merging posteriorly into striae. On some specimens these are parallel to the dorsal border, and oblique in the central portion. On the posterior margins of several operculars the striae break up into tubercles. A few operculars have closely spaced tubercles over much of the surface. The internal surface is smooth.

Visceral skeleton

The palatoquadrate complex, best seen on K. U. no. 9939 (fig. 4), consists of endopterygoid, ectopterygoid, metapterygoid and quadrate. No trace of epipterygoids, dermopalatines or autopalatines, such as Moy-Thomas (1937:392, fig. 5) described for Rhabdoderma, has been observed.

The endopterygoid has a long, ventral, anteriorly-directed process, and an anterodorsal process that meets the metapterygoid in forming the processus ascendens. The suture between the endopterygoid and metapterygoid, seen in lateral view, is distinct in some specimens and has an associated ridge; these bones appear to be fused in others, without regard to size. This suture curves dorsally from a point anterior to the quadrate and passes anterodorsally to the extremity of the processus ascendens. The suture is visible on the medial side only near the processus ascendens, for it is covered by a dorsal, toothed extension of the endopterygoid. The endopterygoid has a smooth lateral surface; the medial surface is covered with tiny granular teeth, in characteristic "line and dot" arrangement. The teeth extend onto the ventral surface of the ventral process.

Two long, narrow, splintlike bones covered on one surface with granular teeth are interpreted as ectopterygoids. These are 13.0 and 16.0 mm. long and each is 1.5 mm. wide. Orientation of these is unknown, but they probably fitted against the ventral surface of the ventral process of the endopterygoid (Moy-Thomas, 1937:fig. 5).

The metapterygoid has a smooth surface in both views. The dorsal edge has a thickened, flared margin that presumably articulated with the antotic process of the basisphenoid. No articular surface for the basipterygoid process has been observed.

The quadrate is distinct and closely applied to the posteroventral margin of the complex. In medial view the margin is nearly straight and continues to the ventral edge. The ventral surface is thickened and forms a rounded, knoblike articular surface. In lateral view the surface is smooth; the anterior margin is irregular (or perhaps broken on all specimens), and proceeds in an irregular convex curve from the posterior to the ventral margin.

The general shape of the palatoquadrate complex is most nearly like that of Rhabdoderma elegans (Moy-Thomas, 1937:fig. 5). The orientation of the complex in the living fish was probably oblique, with the processus ascendens nearly vertical, the quadrate oblique, and the ventral process of the endopterygoid extending dorsoanteriorly and articulating with the parasphenoid.

Of the hyoid arch only the ceratohyals (see fig. 5A) are preserved in several specimens. These are long, curved bones with a posteroventral process and widened, flaring posterior margin. The medial (?) surface is concave in one specimen. The lateral (?) surface displays a distinct ridge on several specimens, arising on the dorsal surface opposite the posteroventral process and extending diagonally to the anteroventral end of the anterior limb. The impression of one other specimen appears to have a central ridge because of greater dorsal thickness and narrowness. Both surfaces are unornamented.

The urohyal (see fig. 5B) is an unornamented, Y-shaped bone, with the stem of the Y pointing anteriorly. Orientation with respect to dorsal and ventral surfaces is uncertain. In one view a faint ridge, also Y-shaped, occurs on the expanded posterior portion, and the surface is convex. The anterior process has a convex surface, sloping evenly off to the lateral margin; the opposite side of the process has a concave surface. The posterior portion has a slightly depressed area (see fig. 5B) at the junction of the "arms" of the Y.

The five branchial arches are represented by the ceratobranchials, several of which are preserved on K. U. no. 11431. These are long bones with anteriorly curving ventral ends. The medial surfaces are partly covered with minute granular teeth; only the dorsal part is without teeth. The dorsal articular surface is convex dorsally and rounded.

The mandible (fig. 3), the best specimens of which are K. U. nos. 788 and 11425, is seen only in lateral and ventral views, with only angular, splenial and dentary visible.

The angular forms the main body of the mandible, and is similar to that of Spermatodus. The dorsal margin of the angular is expanded in the central region, with some variation. One specimen has an expanded portion slightly anterior to that of the opposite angular. The articular surface near the posterior end has not been observed; the posterior end of the angular slopes off abruptly. The anterior sutures are seen in only two specimens, K. U. nos. 788, 11425. The dentary meets the angular in a long oblique suture; the dentary gradually tapers posterodorsally and ends on the dorsal surface of the angular. The splenial fits into a posteriorly directed, deep V-shaped notch on the ventral surface. The lateroventral surface of the angular contains sensory pores of the mandibular line. The ventral surface extends medially into a narrow shelf, approximately 1.0 mm. wide, which extends the full length of the bone; the external surface of this shelf is smooth and slightly concave dorsally. Ornamentation of the angular consists of tubercles and longitudinal or oblique striae, occurring mostly on the expanded portion. The medial surface is not seen. Several broken specimens show a central canal filled with a rod of calcite; in one of these the sensory pores are also calcite-filled and appear to be connected to the rod. Thus the pores originally opened into a central canal.

The dentary is an unornamented bone with the anterior half curving medially; the greatest height is anterior. This bone in specimen K. U. no. 11425 bears irregularly spaced, simple, recurved, conical teeth; nine were counted, but there is space for many others. One other specimen, no. 11429, seems to have tiny tubercles on the surface. The dentary meets the splenial dorsally in a straight suture.

The splenial also curves medially, and as stated, meets the dentary in a straight suture. Ornamentation on this bone was not observed. The posterior margin is V-shaped and fits the notch in the angular. The ventral surface bears three or more sensory pores of the mandibular line.

The gular plates are oval. The medial margin is straight to slightly curved, the lateral margin curved crescentically, the posterior end is blunt, and the anterior end somewhat rounded. Ornamentation varies greatly; some bones show only a few tubercles, whereas others exhibit an almost concentric pattern of closely spaced striae. Typically there are some tubercles in the anterior quarter or third of the total length; these pass into longitudinally oriented striae in the posterior section. A few have only randomly oriented, widely-spaced striae. The internal surface is smooth.

The coronoid (K. U. no. 11428) is a triangular bone, with the apex pointing dorsally. The lateral surface is smooth; no teeth were observed. Moy-Thomas (1937:292, 293) mentions several tooth-bearing coronoids in Rhabdoderma, but as yet these have not been seen in Synaptotylus.

Axial skeleton

Only three specimens (K. U. nos. 786F, 787F, 11450) show parts of the vertebral column, but isolated neural and haemal arches are numerous. All are of the coelacanth type, having Y-shaped neural and haemal arches, without centra. A total count of 38 was obtained, but this was incomplete; the actual number was probably near 50. Counts of 10 and 16 haemal arches were obtained in two of the specimens. Total height of neural arches ranges from 7.5 to 12.0 mm., and of haemal arches, from 9.0 to 12.0 mm. The shorter arches are anterior and the height increases gradually to a maximum in the caudal region. Height of the spines varies from 4.0 to 9.0 mm., or from twice the height of the arch in the anterior to three times the height in the caudal region. Total width of the base, measured in isolated specimens because lateral views in other specimens prevented measuring width, ranges from 0.7 to 4.2 mm. The short, broad arches having short spines occur at the anterior end of the spinal column; the narrower arches having tall spines occur toward the caudal end. Broken neural and haemal arches show a thin covering of bone with a central, calcite-filled cavity, which in life may have been filled with cartilage (Stensioe, 1932:58, fig. 20).

No ossified ribs have been observed, either isolated or in place.

For further description of the axial skeleton, see Hibbard (1933).

Girdles and paired fins

A nearly complete pectoral girdle on specimen K. U. no. 11433 (see fig. 6A) has only a cleithrum and clavicle. No evidence of an extracleithrum or supracleithrum has been observed, but the extracleithrum may be fused to the cleithrum. The two bones form a boot-shaped unit, with the anteroventral part turned medially to form a horizontal process which meets the opposite half of the girdle. In lateral view the surface is unornamented, and convex in the ventral half. The suture between the cleithrum and clavicle begins on the expanded posterior portion, the "boot-heel," at a point immediately below the greatest width on the posterior margin, passes anteriorly, then turns sharply and parallels the anterior margin. The shape of the cleithrum resembles that in Rhabdoderma and the internal surface is not ridged (see Moy-Thomas, 1937:fig. 9). The exact orientation in the fish is uncertain, but if the median extension is really horizontal, then the posterior expansion is directed caudally. The medial surface is concave, steepest near the anterior margin, and then slopes outward evenly. In medial view one specimen (K. U. no. 11426) shows a small, caudally directed projection of bone, evidently for articulation of the fin-skeleton, at the widest portion of the cleithrum. Sutures on several specimens were indistinct. Broken specimens show sutural faces, but many nearly complete specimens show little or no indication of sutures, without regard to size of the girdles. The internal structure of the fin was not observed.