164 SILURIAN TRILOBITES. 



Localities. — Caradoc Rocks. Desertcreat, &c., Tyrone, Mus. P. Geology. The 

 species ranges all through the North American Continent, in Lower Silurian rocks. 



AsAPHUs (IsoTELus) AFFiNis, M'Coy. PI. XXIV, figs. 13 — 14. 



IsoTELUS AFFiNis, M'Coy. Svnops. Foss. Woodw. Mus., pi. i f, fig. 3, 1851. 

 AsAPHUS — Salter. App., Ramsay Geol. N. Wales, Mem. Geol. Surv., vol. iii, 



p. 310, pi. viii, fig. 15 (pi. xii, fig. 4?), 1866. 



j]. {Isot.) longe ovatus, capite rotundato, spicuUs brevibus. Oculi submedii pawi. 

 Sutura facialis ad frontem marginalis. Axis caud<s l(Bvissim(je longus, et apice vix 

 prominulo tenus prominens. 



There are possibly two species in the Upper Tremadoc Rocks, and I retain for one of 

 them the name given by Prof. M'Coy to an imperfect specimen, without head and 

 without apex to the tail, but which, on comparison, seems to agree (the figure does not) 

 with our Tremadoc species. In case the J. Homfrayi, fully described by me in the 

 volume of the ' Memoirs' above quoted, should prove to be identical with this, Prof. 

 M'Coy's name must give way, as it is quite impossible to determine his specimen. Still, 

 the great difi"erence in the facial suture in front, rendering this species a sort of link with 

 Basilicus, ought to distinguish it from the next, to which it has the closest resemblance. 

 Were the two alive, we should probably find them totally different in colour. But 

 palaeontologists have no such advantages in pursuing their studies, and must make the 

 most they can of form. 



Oblong-oval, two and a half inches long, of which the head measures more than a 

 third, and is rounded in front ; the glabella is very obscurely marked out, and has no 

 furrows -. it is most convex forwards. The facial suture curves boldly out above the eye, 

 and cuts the margin considerably outside of it. Below the eye, which is of moderate 

 size, more than half-way up the cheek, and close to the glabella, — the suture again curves 

 largely out. No distinct neck-furrow is to be seen ; and this is usual in Isotelus. 



Thorax shorter than the head, but as long as the caudal shield. The axis broader 

 than the sides, and not strongly separated from them. Pleurae flat as far as the fulcrum 

 (which is at one-half in the middle rings), then curved rather than bent down. The 

 facet broad and very well marked. Pleural groove obscure, except just beneath 

 the fulcrum. 



Caudal shield larger than a semicircle, but proportionately rather short, without 

 any segmental furrows on the sides, except the uppermost. Axis narrow, tapering, well 

 defined, and reaching three fourths the length of the shield; its end not prominent. 

 Incurved fascia broad, striate. 



The under side of the head has the broad epistome well divided by a strong vertical 



