OSTRA CODERMS 



6 7 



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, " Scaphaspis" Closely 

 related is the American Pteraspid, Palceaspis (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 



