The True Sharks 189 
contracted than in the Asiastic species, called Heterodontus. 
For this reason Dr. Gill has separated the former under the 
name of Gyropleurodus. The differences are, however, of slight 
value. The genus Heterodontus first appears in the Jurassic, 
where a number of species are known, one of the earliest 
being Heterodontus falctfer. 
The discussion of the long array of fossil Heterodontide and 
allied families may be here omitted. It is an interesting fact 
that the only sharks known to exist in the Triassic period 
belong to this family from which all recent sharks are descended. 
Very lately the discovery has been made that in sharks of 
this group a rudiment of a sixth gill-segment exists. This 
demonstrates a close relation to the Notedant. 
Suborder Galei—The great body of recent sharks belong to 
the suborder Galei, or Euselachit, characterized by the astero- 
spondylous vertebre, 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 Scylliorhinide, Lamnide, and Carchariide 
(Galeorhinid@). 
The Asterospondyli are less ancient than the preceding groups, 
but the modern families were well differentiated in Mesozoic 
times. 
Among the Gale: 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 Scylliorhinide.— The most primitive of the modern 
families is doubtless that of the Scylliorhinide, 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 
