164 
HETEROSTRACI. 
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-seales 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 Scapkaspis 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 manner, 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. Pay Lankester , 1873. 
The undermentioned ventral shields are probably referable to 
Pteraspis rostratci. 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 
Scaphaspis 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 P>id. p. 150, pi. i. b. figs. 9-11. (ft* J - 
3 Haidinger’s Haturw. Abkandl. vol. i. (1847), p. 159, pi. v. 
4 E. R. Lankester, Rep. Erit. 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. 
Reichsanst. vol. vii. pt. i. (1874;, p. 49, pi. ii. fig. 2. 
