SELACHII. 41 



Sub-Order 1. Aster ospondyli. 



These are the "sharks" and "dog-fishes" of the existing 

 fauna. Specialization results in no marked depression of the 

 body, and the pectoral fins never grow forward towards the 

 head ; the spiracles tend to diminish and are often absent. 

 The anal fin is always present. The vertebral centra, when 

 fully developed, are strengthened by longitudinal ridges or 

 radiating laminae, which, when viewed in transverse section, 

 are shown to predominate over the concentrically-disposed 

 laminae. 



All the families of this sub-order are characterized by the 

 presence of two dorsal fins, except the Notidanidae, which 

 possess only a single dorsal fin and in general occupy a unique 

 position. One living species of Notidanus (Heptanchus) some- 

 times exhibits feebly asterospondylic vertebrae in the caudal 

 region, and the teeth of the earliest known species (fig. 32 B) 



FIG. 32. 



Outlines of teeth of (A) Hybodus polyprion from L. Jurassic, (B) Notidanus 

 daviesi from U. Jurassic, (c) Notidanus microdon from U. Cretaceous, and 

 (D) Notidamts glgas from Pliocene ; nat. size. 



are almost identical with those of certain forms of Hybodus 

 (fig. 32 A). There is thus some reason for placing the Noti- 

 danidae with the primitive Asterospondyli, and it is unfortunate 

 that so little is known of their extinct representatives. A 

 typical skeleton of Notidanus itself has been discovered in 

 the Upper Jurassic (Lithographic Stone) of Bavaria, and 

 numerous detached teeth have been obtained both from this 

 and subsequent formations. These teeth are of peculiar 

 interest as showing how they were at first firmly fixed and 

 then gradually assumed their modern characters. The Jurassic 



