Aster ospondyli. 



21 



of Lyme Regis arid the Wealden of Pevensey Bay, Sussex. 

 Palceospmax, from the Lower Lias of Lyme Regis, and the 

 Upper Lias of Wiirtemberg, is a small fish with smooth dorsal 

 fin-spines and simple constricted vertebrae; Synechodus, of 

 Cretaceous age, is almost identical. Acrodus, ranging from the 

 Muschelkalk to the Grault, only differs from Hybodus in the less 

 cuspidate character of its teeth (Fig. 122). Aster acanthus, with 

 a dentition commonly named Stropliodus (Fig. 124), is proved by Table-case 

 specimens in the collection to differ only from Acrodus in the No. 39. 

 pattern on its teeth and fin-spines. Fine examples of its head- 

 spines (Sphenonchus) , from the Oxford Clay of Peterborough, 

 are exhibited; while a jaw in a block of Great Oolite from Caen, 

 Normandy (Fig. 124), is unique. It will be observed that the 

 front prehensile teeth in Hybodus, Acrodus, and Asteracanthus 

 {Stropliodus), are relatively larger and less numerous than those 



FIG. 124. Jaw of Asteracanthus (Strophodus medius, Owen); Great Oolite, Caen, Normandy. 



of Gestracion. The latter genus seems to range from the Upper 

 Jurassic to the present day. 



An interesting Carboniferous family of which little is known Table-case, 

 beycnd the dentition, is that of the Cochliodontidae, apparently No> 3O - 

 closely related to the Cestracionts. Their jaw is arranged 

 somewhat like that of Cestracion, but the several series of 

 lateral teeth are each represented by a single plate, coiling 

 inwards by growth at the outer edge. Cochliodus (Fig. 125) 

 is the typical genus, and Streblodus, Psephodus, Sandalodus, 

 Poecilodus, etc., are very similar forms. Many of the teeth 

 named Helodus pertain to the symphysis of the jaw of these 

 fishes ; and in one genus, Pleuroplax, from the Lower Carboni- 

 ferous and Coal Measures, such teeth are only imperfectly fused 

 together in the plates. 



