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 (¢.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 
tim might become apposed, as in the Cyclostomes. 
