532 The True Sharks 



times at the sides of the head. In some cases the teeth remain 

 as primitive shagreen cusps on the rim of the mouth, but be- 

 come heavy and bluntish behind; in other forms the fusion 

 of tooth clusters may present the widest range in their adapta- 

 tions for crushing; and the curves and twistings of the tri- 

 toral surfaces may have resulted in the most specialized forms 

 of dentition which are known to occur, not merely in sharks 

 but among all vertebrates." 



In this neighborhood belongs, perhaps, the family of Tamio- 

 batidce, known from the skull of a single specimen, called Tamio- 

 batis vetustus, from the Devonian in eastern Kentucky. The 

 head has the depressed form of a ray, but it is probably a shark 

 and one of the very earliest known. 



Suborder Galei. The great body of recent sharks belong to 

 the suborder Galei, or Euselachii, characterized by the astero- 

 spondylous vertebrae, each having a star-shaped nucleus, and 

 by the fact that the palato-quadrate apparatus or upper jaw 

 is not articulated with the skull. The sharks of this suborder 

 are the most highly specialized of the group, the strongest and 

 largest and, in general, the most active and voracious. They 

 are of three types and naturally group themselves about the 

 three central families Scyliorhinidce, Lamnida, and Carchariidcs 

 (Galeorhinidce) . 



The Aster os pondyli are less ancient than the preceding groups, 

 but the modern families were well differentiated in Mesozoic 

 times. 



Among the Galei the dentition is less complex than with 

 the ancient forms, although the individual teeth are more 

 highly specialized. The teeth are usually adapted for biting, 

 often with knife-like or serrated edges; only the outer teeth 

 are in function ; as they are gradually lost, the inner teeth are 

 moved outward, gradually taking the place of these. 



We may place first, as most primitive, the forms without 

 nictitating membrane. 



Family Scyliorhinidae. -- The most primitive of the modern 

 families is doubtless that of the Scyliorhinida, or cat-sharks. 

 This group includes sharks with the dorsal fins both behind 

 the ventrals, the tail not keeled and not bent upward, the 

 spiracles present, and the teeth small and close-set. The species 



