Pfagiostomi — Sharks and Bays. 



11 



understand that many so-called species founded on different forms 

 of fossil crushing teeth found detached from one another, may, 

 after all, have really belonged to different parts of the mouth of 

 one and the same individual species of Shark. 



The dentition of Acrodus differs from that of Ccstracion mainly 

 in the replacement of the numerous small symphysial teeth by two 

 or three rows of large teeth, less pointed, as is shown by several 

 Liassic fossils in the case. A nearly complete jaw of Strophodus Table-case, 

 (Fig. 23), from the Great Oolite of Caen, Normandy, proves that No - 29 « 



Fig. 23.— Jaw of Strophodus medius, Great Oolite, Caen, Normandy. 



in this genus there was also a similar divergence from the surviving 

 type. Acrodus ranges from the Muschelkalk to the Cretaceous 

 inclusive, and Strophodus is Jurassic and Cretaceous ; but in the 

 latter formation it is difficult to distinguish both these genera from 

 Drepanephorus, which is a characteristic Shark of the Chalk. The 

 smooth dorsal spines of this fish were originally described by 

 Agassiz as Spinax, and the genus was erroneously referred to the 

 Spinacidee by Egerton. Wodnika, from the Permian Kupferschiefer, 

 is also perhaps an early Cestraeiont. 



The Cochliodontida are an interesting Carboniferous family known Table-cases, 

 only by their dentition, which is arranged ^ os - ^9, 30. 



somewhat like that of Cestracion, but in 

 which the several series of lateral teeth 

 are each represented by a single plate, 

 coiling inwards by growth at the outer 

 edge. Cochliodus (Fig. 24) is the typical 

 genus, and Strejblodus, Psephodus, Sanda- 

 lodus, Pcecilodus, etc., are very similar 

 forms. Many of the teeth named Udodus 

 almost certainly pertain to the symphysis 

 of the jaw of these fishes ; but JTelodus 

 simplex, from the Coal-measures, is a distinct genus. 



Fig. 24. — Teeth of Cochliodus 

 contortus, Carboniferous Lime- 

 stone, Armagh. 



