﻿164 , JIETEROSTRACI. 



the fish, showing the scar for the insertion of the spine at 

 the hinder extremity of the shield. To the present writer, 

 however, recent discoveries on the Continent suggest 

 another and more probable explanation of the specimen. 

 The scales appear to be referable to the flank, the so-called 

 dorsal ridge-scales of Lankester not being so clearly dis- 

 tinguished from the other scales as indicated in the original 

 description, and being truly referable to the middle of the 

 flank. The supposed " scar " for the insertion of the spine 

 seems to the present writer to be the line of separation 

 between the dorsal and ventral shields • and a small pos- 

 tero-lateral portion of the dorsal shield remains above, 

 while the greater portion of the hinder extremity of the 

 ventral shield is preserved below. The latter exhibits the 

 broad, longitudinal median convexity characteristic of the 

 so-called Scapliaspis lloydi, and this is continued back- 

 wards by a series of calcifications which have much more 

 the appearance of ridge-scales than those determined as 

 such by Lankester. In this manuer, the re-entering angle 

 at the hinder margin of the armoured portion of the body 

 is more easily explained than by the accidental crushing 

 required in the original interpretation of the fossil. 

 Moreover, the slight longitudinal mark on the fragment of 

 shield bounding each margin of the supposed dorsal "scar " 

 is paralleled by a similar mark observed near the lateral 

 margin of several shields both of Pteraspis and " Sca- 

 phaspis," while the median portion of the inner surface of 

 the shield is always smooth and even. 



Presented by Prof. E. Ray Lankester, 1873, 



The undermentioned ventral shields are probably referable to 

 Pteraspis rostrata. They were described by Agassiz under the 

 names of Cephalaspis lewisii 1 and C. lloydii 2 , subsequently made 

 the type of Pteraspis by Kner 3 , and finally regarded as the type of 

 Scapliaspis by Lankester, under the name of S. lloydii 4 . 



P. 5861. Internal cast of a fine, uncrushed specimen, 0*125 in length, 



1 Poiss. Foss. vol. ii. pt. i. (1835), p. 149, pi. i. b. fig. 8. 



2 Ibid. p. 150, pi. i. b. figs. 9-11. 



3 Haidinger's Naturw. Abhandl. vol. i. (1847), p. 159, pi. v. 



4 E. E. Lankester, Rep. Brit. Assoc. 1864, Trans. Sect. p. 58, and Fishes Old 

 Red Sandst. pt. i. (Pal. Soc. 1868), p. 20, pi. i., pi. vii. figs. 1, 6, 17, 18. A 

 shield, wanting the posterior extremity, from the Upper Silurian of Iwanie, 

 Galicia, is assigned to this "species" by A. von Alth, Abhandl. k. k. geol. 

 Keichsanst. vol. vii. pt. i. (1874;, p. 49. pi. ii. fig. 2. 



