OSTRACODERMS 67 
nection they may be described, if only to indicate that 
they are in no way closely connected with the ancient 
shark types (p. 78), and that they are accordingly of but 
indirect interest in the descent of jaw-bearing vertebrates. 
Ostracoderms may readily be reduced to three general 
types, Pteraspid, Cephalaspid, and Pterichthid. The first, 
oldest, and probably simplest occurs in the Lower Old 
Red Sandstone of Herefordshire. It was provided with 
arched back and breastplate (Figs. 74, 76, 77), from whose 
anterior lateral notches a pair of eyes protruded ; the sur- 
face of these plates (Fig. 77) appears to have been grooved 
for sensory canals. Pteraspis, as seen in the restoration; 
had a snout plate, a dorsal spine, and a body casing of 
rhomboidal scales ; its mouth was probably in the region 
immediately below the eyes, in front of the margin of the 
well-rounded ventral plate; this was generally regarded as 
the dorsal plate of a kindred genus, “ Scaphasfis.” Closely 
related is the American Pteraspid, Palegaspis (Claypole), 
from the Upper Silurian of Pennsylvania (Fig. 75); this 
form lacks the dorsal spine of the English species; it has a 
well-marked lateral plate intervening between those of the 
back and ventral side, and, according to its discoverer, 
Professor Claypole, possessed pectoral fins similar to those 
seen in Fig. 123. Its hinder trunk region is unknown. 
Cephalaspis, the second type of Ostracoderm, is from 
the Old Red Sandstone of Scotland (Figs. 78, 79). It was 
curiously suggestive of a trilobite, and with little doubt 
mimicked this ancient crustacean in its life habits. Its 
most prominent feature is a crescent-shaped head, with 
sharp rounded margin like a saddler’s knife. This is 
protected dorsally by but a single plate, arching upward 
and backward; at its summit was a pair of closely apposed 
eyes, and near its flattened rim were pouch-like sensory 
