CEPHALASPID^. 33 



disposed concentrically round a middle point, as in Ft. CroucJtii, but this portion of the 

 shield is apparently elevated into a slightly convex boss. The surface-striations of Pt. 

 rostratus differ considerably from those of Pi. Crouchii. They are very fine, the 

 groovings being only j^j^th of an inch apart near the centre of the disc, and leaving a 

 rounded dull ridge with crenate margins between them. 



Localities. — This species occurs abundantly at Cradley, near Malvern, Herefordshire, 

 and in beds of the same horizon at Whitbach and in the neighbourhood. It is found 

 associated with Scaphaspis Lloydii. 



3. Pteraspis MiTCHELLi, PowHe. PI. V, figs. 1, 2, 6, 10, 11. 



Pteraspis MiTCHELLi, Poivrie. Geologist, vol. vii, pp. 170 — 172, 1864. 



— — Lan/cester. Brit. Assoc. Report, Trans. Sect., p. 58, 1864. 



1 have not had specimens of this species sufficiently perfect to enable me to charac- 

 terise it properly. The specimen figured (PI. V, figs. 6, 10) was briefly noted by Mr. 

 Powrie in the ' Geologist,' in 1864, and an outline-sketch was given. The lateral cornua 

 are not seen, but the disc, rostrum, and spine are, to a certain extent. The disc appears 

 to be intermediate in form between that of Pt rostratus and Pt. Crouchii. The specimen 

 and others even less well preserved were obtained by Mr. Powrie from quarries in 

 Forfarshire, where Heterostraci had been discovered for the first time in Scotland by 

 the Rev. Hugh Mitchell. 



A few rhoraboidal scales also obtained by Mr. Powrie from this locality (PI. V, fig. 1) 

 probably belong to this or an allied species. 



