THE JURASSIC PERIOD. 



2 4 I 



Fishes are still represented by Ganoids and Placoids. The 

 Ganoids, however, unlike the old forms, now for the most 



Fig. 173. A, Restoration of the animal of the Belemnite ; B, Diagram showing the 

 complete skeleton of a Belemnite, consisting of the chambered phragmacone (ft), the 

 guard (b), and the horny pen (c) ; C, Specimen of Belemnites canalicitlatus, from the 

 Inferior Oolite. (After Phillips.) 



part possess nearly or quite symmetrical (" homocercal ") tails. 

 A characteristic genus is Tetragonolepis (fig. 174), with its 



Fig. 174. Tetragonolepis (restored), and scales of the same. Lias. 



deep, compressed body, its rhomboidal, closely-fitting scales, 

 and its single long dorsal fin. Amongst the Placoids the teeth 



