Part 11

We have succeeded to prepare a horizontal section of the macula. This is oval, perfectly homogenous and without any structure proper, showing only indistinctly a mottled medley of pale, brownish spots in the clear mass. There is in both maculæ a small dark, angular spot placed in their inferior part, fig. 9.

=Centropleura= ANGELIN.

This author has delineated most distinct maculæ in Centr. Steenstrupi pl. III fig. 4, b. two, small elongate darkish spots, near the posterior margin. I have not seen any specimen of this trilobite and have consequently not been able to follow up ANGELIN'S observation.

=Chasmops= M'COY.

Pl. III fig. 10.

In his work on the Russian Phacopidæ FRIEDR. SCHMIDT has given the figures of Chasmops Eichwaldi and Ch. Wesenbergensis showing, as it were, patches of black maculæ. But as there is no mention of them, and as they are quite unlike the macula? which are described here, I am in doubt of their real nature. We have figured a macula from the hypostoma of a Chasmops, probably Ch. macroura, from a drift block near Rostock. It is elongate and narrow, more than four times longer than broad. Its smooth surface is a little scooped out and no structure is observable.

=Chirurus= BEYRICH.

We find the hypostomic maculæ annotated and delineated as occurring in the following species.

Ch. bimucronatus SALTER pl. 5, fig. 5 as two narrow elongated pits, surface seems intact. No mention of them in the letter-press.

Ch. exsul SCHMIDT pl. VI fig. 9. Indications of the oblong, narrow maculæ.

Ch. macrophthalmus SCHM. pl. VII, figs. 1 c, 2. Faint indications of narrow pits.

Ch. Quenstedti BARR. Tab. 42 fig. 3. Maculæ near lower border as two narrow slits.

Ch. spinulosus SCHM. pl. VII fig. 10. The lateral pits well visible.

Ch. tumidus SCHM. pl. VIII, fig. 22. The pits are short, but present.

The exterior surface of the hypostoma is entirely covered with small wartlets, closely set, of different size in the various species, and spread between them lie a great number of elongate smooth spots dispersed, on the interior side of the hypostoma showing an excavated surface (pl. III fig. 16).

=Chirurus (Cyrtometopus) clavifrons= DALM.

Pl. III figs. 17-21.

The shape of the hypostoma is shown in fig. 21 pl. III. It is regularly shield-formed with posterior margin rounded. A shallow groove running parallel with the margins forms a large, faintly elevated border. The anterior margin projects on both sides in a short, backwards directed, broad and flat wing, hollowed out with a pit on the front side, and there is a posterior lateral wing, midways between the anterior and posterior margin short, blunt, directed obliquely backwards. The whole exterior surface is covered with diminutive tubercles, the above mentioned smooth spots interspersed on the central disk. A little below the middle there are two lengthened pits, so shallow that they often are not discernible. In a specimen of Chirurus spinulosus NIECZKOWSKI from Estland there are two, almost 3 mm. long dots, one on each side above the maculæ as the fig. 21 shows and in several others of this generic group there are also indications of similar. I cannot compare these dots with anything more than the lengthened spots visible in Acidaspis crenata.

The maculæ are wanting in this species, but the cephalic eyes are well developed. A section of them fig. 19 shows the enormous difference of the shell in the eye and the surroundings where the shell surpasses it many times in thickness. The eye consists in fact only of the ovate, beadlike lenses of which a string is seen sectioned in the figure mentioned. More enlarged (fig. 20) a nucleus is visible in each lens, and in a horizontal section (fig. 18) a little below the surface, where they are more pressed against themselves, they have a polygonal shape. The figure 17 seems to represent lenses that have been much changed interiorly, having only a narrow zone left of the primary structure.

=Chirurus glaber= ANGELIN.

Pl. III fig. 11.

As a sample of the shape of the surface of the eyes in this genus, we have given a figure, showing the small rounded lenses.

=Chirurus ornatus= DALMAN.

Pl. III figs. 12-14.

The cephalic eyes have globular lenses, of more than double the size of those in Chir. clavifrons. The difference between the lenses and the surrounding shell amounts at the highest to thrice the former, while in Ch. clavifrons it is at least seven times as much. In a horizontal section the lenses have a little, darkish irregular nucleus surrounded by a radiated structure (fig. 12).

The maculæ (fig. 14) are slightly concave, oblong, smooth, surrounded by comparatively large granulations.

=Chirurus speciosus= DALM.

Pl. IV fig. 1.