244 



ZOOLOGY 



SECT. 



ORDER 2. OSTEOSTRACI. 



Cephalaspis (Fig. 867) may be taken as an example of the five genera included 

 in this order. The head is covered with a calcified shield, which has a curious 

 resemblance to the cephalic shield of Limulus or of a Trilobite, being gently 

 curved above, produced behind into spines, continued ventrally into a sub-frontal 

 plate (B, s. f. p.), and having a pair of orbits (A, or) for the eyes near the middle 

 of the dorsal surface. Behind the shield, towards the ventral surface, is a plate 

 which probably supported the operculum (C, op.}. The scutes contain some 



or 



FIG. 867. A, restoration of shield of Cephalaspis lyelli, dorsal aspect ; B, diagram of ventral 



s: C, restoration of Cephalaspis murchisoni (Devonian). 

 1 * ' (From the Brit. Mus. Cat. of Fossil 



aspect of shield of Cephalaspis; 



op. opercular plate ; or. orbit ; s. /. p. sub-frontal plate. 



Fishes.) 



lacunae, and therefore approach in structure to bone. The posterior portion of 

 the body is covered by deep, narrow scales ; there is a single dorsal and a hetero- 

 cercal tail fin, but no trace of paired fins is known. 



ORDER 3. ANTIARCHA. 



This group contains five genera, of which Pterichthys (Fig. 868) may betaken as 

 an example. It presents a broad and high anterior region, covered by articulated 

 plates which have the structure of bone and are covered by a layer of enamel, 

 and a caudal region covered by rounded or hexagonal scales. The orbits are 

 placed close together on the top of the head, and between them is a plate pitted 

 on its inner surface, apparently for the pineal body. There is a pair of large 

 pectoral fins (pet. f. ) covered by strong scutes, a single dorsal fin (d. /. ) with 

 fulcra but apparently no fin-rays, and a heterocercal tail-fin (c. f. ). 



