196 



P TERA SPID OMORPHI 



the entire surface of the scales and plates of the Pteraspidae 

 (Fig. 166, E, I). 



Thus we seem to be able to trace a genetic connection between 

 the very aberrant Pteraspids and some early kind of shark, through 

 the Coelolepidae ; but whether the four families given below really 

 form a phyletic series remains doubtful. 



The Pteraspidomorphi are among the most ancient of vertebrate 



FIG. 166. 



Diagrams of the structure and development of the dermal skeleton of A, an early stage, and 

 B, later stages of Elasmobranchs ; C, Thelodtis ; D, Psamrnostev.s ; E, Pteraspis, all in section 

 at right angles to the surface ; the dentine is black. F-I, enlarged views of the outer 

 surface of the dermal skeleton of, F, Thelodus head ; G, Thelodus tail ; H, Psammosteus shield ; 

 and I, Pteraspis shield. 6/j expanded basal plate ; b.m, basement membrane ; c.t, connective 

 tissue ; d.c, dentine cap ; ep, epidermis ; I, bony lamellae ; p, pulp-cavity ; 7', surface ridge ; tr, 

 bony trabeculae of vascular layer. 



animals ; they flourished in Upper Silurian and Devonian times, 

 beyond which they do not appear to have survived. 



Family COELOLEPIDAE. The body is flattened dorso-ventrally, and 

 expanded on each side into a longitudinal flap of considerable size. A 

 single dorsal fin is present. The exoskeleton is in the form of numerous 

 scattered placoid scales ; either sharp, conical, and tooth-like as in 

 Lanarkia (Fig. 167), or more flattened and closely fitting as in Thelodus 

 (Fig. 168). No lateral-line system has been made out (Traquair [466], 

 Kohon [371]). 



Thelodus, Ag. ; Coelolepis, P. ; Upper Silurian and Devonian, Europe. 

 Lanarkia, Traq. ; Upper Silurian, Scotland. 



