PTERICHTHYS AND CEPHALASPIS 



69 



organs. The angles of the head plate are in some genera 

 produced most acutely, and bear spines which served prob- 

 ably in progression. The body walls were encased in 

 metameral derm plates, which became arched in the 

 median line to serve as a dorsal fin. A heterocercal tail 

 and an anal fin were also present. Problematical opercu- 

 lar flaps protruded at the sides of the head plate, and 

 represented (as is now known) a continuation of the elastic 

 middle layer of the head plate. 



Pterichthys must be looked upon as the culminating 

 type of these anomalous forms (Figs. 80-82). As in some 

 Cephalaspids, there are two body regions that are cui- 

 rassed, — head and thorax. The tail portion is encased 

 in dermal plates ; it bears a dorsal fin and a clumsy 

 heterocercal tail. In the consolidation of its armoured 

 parts the elements are usually clearly indicated. The 

 curious arm-like jointed appendages at the lateral head 

 angle were formerly regarded as homologous with the 

 opercular flaps of Cephalaspid, but are now known to be 

 nothing more than the lateral head angles produced and 

 specialized {i.e. jointed for locomotion). The strengthen- 

 ing spine of the dorsal fin is also but a primitive speciali- 

 zation of the body integument ; it is formed by a pair of 

 the bent scales of the dorsal ridge, and is not, therefore, 

 homologous with the radial fin cartilages of fishes. 



In Cephalaspids and Pterichthids there occurs a pineal 

 plate (or its equivalent) which may have been either 

 movable or fixed. In this are to be found the paired eyes 

 and the socket of a median unpaired eye (.-*). In all of 

 these singular forms mouth parts* are wanting. In 



* Smith Woodward has since described a pair of inturned labial plates in 

 the mouth of Pterichthys. Their position suggests that the sides of the mouth 

 rim might become apposed, as in the Cyclostomes. 



