FOSSILS AND PEDIGREES 



349 



character at once distinguishing them from the Selachians, but 

 the underlying skull and brain are essentially Shark-like. A 

 curious feature is the presence of a joint or hinge in the neck 

 region, the armour of the head and trunk being quite distinct, 

 and the head consequently freely movable on the body. The 

 Arthrodires have a somewhat large mouth, armed with for- 

 midable crushing or cutting teeth. Pelvic fins can be seen in 

 some specimens, but the pectorals may or may not have been 

 developed. Like many other archaic fishes, the front part of 



Fig. 125. RESTORATIONS OF PLACODERMS. 



A. Bothriolepis canadensis, X i. (After Patten); b. Coccosteus decipienSyX \. 

 (After Jaekel) ; c. Pterichthys milleri, X i^. (After Traquair.) 



the body is heavily cuirassed, but the hinder part is quite 

 unprotected. Coccosteus is perhaps the most familiar genus, a 

 comparatively small fish, occurring in Europe and particularly 

 abundant in the Old Red Sandstone of Scotland (Fig. 125B). 

 The members of this genus varied in size from about twelve 

 to eighteen inches in length, but certain Arthrodires [Dinichthys, 

 Titanichthys) from North America rival some of the largest of 

 the existing Sharks, attaining to a length of twenty feet or more. 

 The great variation in the form and dentition of these fishes 

 suggests that their habits were equally varied, and the group 



