308 THE PBOCEEDINGS OF THE LINNEAN SOCIETY 



With the exception of a few fishes of a Ganoid character, which 

 appear in some of the upper Silurian strata, the Gestracionts, as 

 they are named by Geologists, are the oldest of known Fishes. 

 Teeth and spines resembling those of the Port Jackson Shark 

 are abundant in the Devonian Rocks of Europe, and they are to 

 be traced all through the Carboniferous and Permian Periods. 

 They are found also throughout the whole of the Mesozoic or 

 Secondary series of Rocks, but are most abundant during the 

 Jurassic period. The teeth of another kind of Shark (Notidanus) 

 occurs during the same period for the first time. It is noticeable, 

 as has been observed by Palaeontologists, that the Jurassic 

 Fauna and Flora of Western Europe were very similar to those 

 of Australia at the present day. Among plants, Cycads and 

 Araucarice ; among fish, the Cestracionts ; among molluscs, Tri- 

 gonia ; and among mammals, Marsupials. 



It may be necessary here to give some explanation of our 

 reasons for rejecting the term Cestracion of Cuvier, which has 

 been extensively used by the most eminent Ichthyologists for 

 many years, and for adopting Blainville's name of Heterodontus, 

 as used by Dumeril, in his Hist. Nat. des Poiss., tome 1, p. 423. 



The word Cestracion (from KeaTpa, a pickaxe, and a%t9, a 

 point), was first used by Klein, in 1742 (Missus tertius, p. 12), 

 as a name for the hammer-headed shark (to which it seems pro- 

 perly to apply), and is now used by Dumeril (Hist. Nat. des 

 poiss. tome, 1, p. 380) to designate the sharks termed by Cuvier 

 Zygaena. Cuvier has also (Regm. Anim. 1817, t. 11, p. 129) 

 given the generic name Cestracion, without assigning any reason, 

 to the Port Jackson shark, although Blainville (Nouv. Bull, des 

 Sciences, p. 121) had a year previously (1816) given to that 

 species the generic name Heterodontus. It seems, then, that not 

 only on the ground of priority, but from the meaning of the 

 respective words, Dumeril is right in adopting Blainville's 

 nomenclature. 



As the family consists of one genus only, the characters of the 

 group are given in the description of the genus. 



