[483] Mem. Geol. Surv. Decade 2, pl. v. p. 2, note.
[484] Isotelus, i. e. equal extremities.
Another division of the Trilobites has the body contracted, and very thick, and the abdomen large and scutiform, without any segmentary divisions; the small crustacean (Illænus perovalis, Murch.) [Lign. 175], fig. 1, will serve to illustrate these characters.
The Trilobite called Bumastus by Sir R. Murchison (from its grape-like form) presents a very curious modification of the normal type. Both the head and caudal extremity are rounded, with no distinct longitudinal furrows; and the whole surface of the carapace is covered by extremely thin, apparently imbricated, lamellæ, the edges of which are undulated, and the intermediate spaces studded with minute dots. The eyes are smooth, and not granulose, as in Calymene. This genus is known in England by the name of the Barr Trilobite, from its occurrence in the limestone near Barr, in Staffordshire; it is sometimes five inches long, and three and a half wide (Geol. Surv. Decade 2, pl. iii. and iv.).
The genus Ogygia (Bd. pl. xlvi. fig. 9) is characterised by the elliptical and depressed form of the carapace, its nearly balanced extremities, and the prolongation of the buckler, or cephalic portion, on each side, into slender spikes, distinct from the body; the thoracic and abdominal regions are divided by two deep, longitudinal furrows, into three lobes; there is also a straight, longitudinal groove, in the front of the buckler (see figures and descriptions of O. Buchii, Geol. Surv. Decade 2). The Trilobites of this genus are found in the Lower Silurian rocks of North Wales and Ireland; they occur also in great abundance in the slate rocks of Angers, and some species are more than a foot in length.
Some species of the genus Phacops have long, pointed caudal appendages, as the P. (formerly Asaphus) caudatus ([Lign. 177]; Bd. pl. xlv. figs. 10, 10′; and Geol. Surv. Decade 2, pl. i.). The eyes are often well preserved, and each contains about 240 spherical lenses.
Lign. 177. Phacops caudatus. Upper Silurian. Dudley.
The caudal extremity.