CEPHALASPID.E. 31 



sides in this part are nearly parallel, converging slightly towards the anterior rounded 

 termination. Where the rostrum becomes excavated, convex and smooth beneath, 

 it broadens very much, and joins on either side the orbital pieces, and in the centre 

 the disc and circular piece. Most casts show well the existence of a sharply marked 

 pit corresponding to this circular bit. The posterior spine in this species is long and 

 strong, its insertion extending into the disc for a distance of about one half of its own 

 length, and one third of that of the disc. The test of the posterior part of the shield 

 is very thick, and gives the perfect shield an outline somewhat different to that of the 

 convex internal cast. 



The cornua appear to be thick processes, overlying the oblique penetration of the 

 shield, as in those of the succeeding species. 



The markings of the internal surface of the disc in most cases afford means of 

 determining otherwise doubtful specimens of Ft. Crouchii. The median line, after the 

 insertion of the spine, is marked by a sharp depression, which becomes fainter towards 

 the anterior portion. On either side of this diverges, from near the centre of the disc, an 

 irregularly marked straight ridge, passing anteriorly in an oblique direction ; sometimes 

 these are not perceptible as ridges. Below these straight .ridges on either side are two 

 curved ridges, the lower pair of which pass from the centre nearly right and left, the 

 upper pair (nearest the straight ridges) curving obliquely towards the anterior margin. 

 In the casts of the interior, which so frequently occur, these ridges are, of course, exhibited 

 as furrows, and the median groove as a ridge. Similar ridges exist in Pt. rostratus, but 

 are not usually so well marked. 



The surface-striations of the shield are exhibited in the drawings. Those on the 

 rostrum are semi-elliptical in direction, the open part of the elhpse being turned anteriorly 

 on the upper, posteriorly on the under surface. 



The groovings are ^th of an inch apart (average), leaving a flat, bright, crenate ridge 

 between them. 



Localities. — Pteraspis Crouchii has been obtained from Whitbach and Ludlow. The 

 greatest number of specimens now occur in that neighbourhood. It is also met with 

 at Abergavenny. I have never heard of specimens of this species occurring in 

 immediate association with Pt. rostratus; and I believe it has a different horizon. 

 Scaphaspis rectus occurs with it and the Cephalaspis asterolepis of Dr. Harley. 



Note. — The scales of Pteraspis (PI. V, figs. 3, 5, 8) which I obtained at Cradley, 

 Herefordshire, perhaps belong to this species. 



