48 PISCES. 



Jurassic) of Bavaria; and another dog-fish, identified with 

 Pristiurus, also occurs in the same horizon. Scy Ilium and 

 Ginglymostoma range from the Upper Cretaceous; and there 

 is an extinct genus in the Cretaceous and early Tertiary, 

 Mesiteia, in which the mucus-canal of the lateral line is 

 supported by calcified incomplete rings resembling the cor- 

 responding supports in Chima3roids. 



FIG. 37. 



Aster acanthus (Strophodus medius) ; imperfect dentition, one-third nat. size. 

 L. Jurassic ; Caen, Normandy. (British Museum, no. 41378.) 



While the Cestracionts are typically Mesozoic, the Lamnidae 

 ;m< I CarchariidaB are essentiallyCainozoic (Tertiary) and Recent. 

 The teeth (fig. 38) are all cuspidate, and the crown is fixed 

 upon a more or less bifurcated base which is much compressed 

 antero-postedorly and thus differs from the stout depressed 

 base in the teeth of the earlier sharks ; the teeth are, indeed, 

 adapted merely for seizing and lacerating prey. Nothing is 

 k;nown of the Lamnida3 earlier than the Upper Cretaceous ; but 

 a Jurassic form of tooth (Orthacodus) hitherto only found 

 detached, is often placed here, and this is remarkable as 

 exhibiting an Odontaspis-like dental crown fixed on a hori- 

 zontally-expanded base. The teeth and vertebrae occurring in 

 the Cretaceous are indistinguishable from those of Odontaspis, 

 Lamna, and Oxyrhina ; but entire fishes found in the Upper 



