Part 17

Pl. V figs. 16-18.

As to its general form the hypostoma resembles that of the preceding species, but there is no border line, which is so characteristic to the former. The maculæ have the same form and are likewise situated on the median line. The cephalic eyes have prismatic lenses with much convex bases, in a transverse section they are hexaedral and of internal radiate structure.

=Niobe= ANGELIN.

The following figures have formerly been given of its hypostoma with maculæ.

N. emarginula BR. II tab. 2 fig. 33.

N. explanata BR. II tab. 2 fig. 35.

N. frontalis BR. II fig. 37.

N. insignis BR. I tab. IV fig. 1 d, 11 tab. 2 fig. 28.

N. læviceps BR. II pl. 2 fig. 34.

The maculæ are distinguished from all other sorts of maculæ, excepting those of Megalaspis, in being, as it were, shelved on a sort of support, formed through the bulging of the hypostomic surface just below them, one such console for each. Their surface lies consequently almost rectangularly to the adjoining surface of the hypostoma. They are white and smooth.

=Niobe frontalis= ANG.

Pl. V figs. 19-21.

The pocketlike consoles are shorter and broader than in N. læviceps and the maculæ also larger and of an elliptic shape. In a horizontal section near the surface the macula has a speckled appearance of black dots, indicating an undeveloped prismatic structure, much more primitive than in the Asaphidæ.

=Niobe læviceps= ANG.

Pl. V figs. 22-26.

It differs from the preceding through longer consoles below the maculæ, which are oval and in a horizontal section exhibit the same spongious structure, pl. V fig. 25. The cephalic eyes consist of polyedric prisms, which through deterioration show a central cylindrical core surrounded by darkish matter like Asaphus. The passage from solid, homogenous prisms to changed ones is most evident in the figure 22.

=Ogygia= BRONGN.

Figures of the maculæ are found in

O. Buchi SALTER, Mem. Geol. Surv., Dec. II pl. VI fig. 3 p. 2, where it is stated that »there are two transverse furrows near the apex, with compressed tubercles between them» There are two narrow crescentic maculæ and between them a little lower a longer, crescentic ridge, which may be the ridge that in other species connects the maculæ. Moreover, the shape of the hypostoma is not concordant with that of the other species. In Monogr. Brit Trilob. pl. 15 figs. 2, 3 there is only a reproduction of the former figure.

O. corndensis MURCH. SALTER in Monogr. pl. 16 fig. 10 interior side of the hypostoma with two lateral maculæ. In the description two pairs of furrows the uppermost is the maculæ. H. WYATT-EDGELL »On the Genera of Trilobites Asaphus and Ogygia and the Subgenus Ptychopyge» in Geol. Magaz. 1867, p. 14, 15 fig. 2 probably a cast with two oblique impressions of maculæ.

O. dilatata (Asaphus) SARS var. Sarsi ANG. SARS in Isis 1835 p. 342, pl. IX fig. 11. The figure has two small maculæ near the posterior margin and the author says »mit einer starken Vertiefung oder Einschnitt an jeder Seite des Endes». ANGELIN pl. XLII fig. 1 b. BRÖGGER'S fig. 38 pl. 3, in II, collies near to the specimen, which is described below, also belonging to Dr. G. HOLM.

=O. dilatata= var. =Sarsi= ANG.

Pl. V figs. 27, 28.

The hypostoma is in so far deviating that the terrace lines have a nearly vertical direction and in this respect much resemble those of the Phillipsiæ and the Proeti. The maculæ, near the exterior angles of the border groove are pear- or spoon-shaped with the apex directed downwards and outwards, and in consequence the longitudinal axis quite opposite to that of the usual direction. The surface is smooth, a little concave. No structure has been discovered. From the pointed apex a slightly elevated ridge runs along the posterior border of the hypostoma and joins the apex of the other macula.

=Paradoxides= BRONGN.

The oldest record of any hypostoma at all amongst the trilobites is that given by WAHLENBERG in his Petref. Suecana (1818) p. 37 Tab. 1, fig. 6, which he, however, considered as the head of the trilobite which he named Entomostracites bucephalus. Figures with maculæ have been given of

Par. Davidis SALTER Mem. Geol. Survey Dec. XI pl. X, fig. 3, a fine figure of a hypostoma with two large oblique maculæ. LINNARSSON in »de undre Paradoxideslagren vid Andrarum» pl. II fig. 2 delineates a gigantic hypostoma with two crescentic tubercular maculæ, having 10 mms. in length.

Par. Forchhammeri ANGELIN Pl. II fig. 3. BRÖGGER, Paradoxidesskiffr. vid Krækling tab. II fig. 10 two marks somewhat different from ANGELIN'S figure. LINNARSSON 1. c. pl. I figs. 9, 10 two hypostomas with maculæ.