THE CRETACEOUS PERIOD. 



275 



the Cretaceous series ; but it disappears in the upper portion 

 of the series, and its place is taken by the nearly-alHed genus 

 BelemniteUa (fig. 205), distinguished by the possession of a 

 straight fissure in the upper end of the guard. This 

 also disappears at the close of the Cretaceous 

 period; and no member of the great Mesozoic 

 family of the Belemmtidce has hitherto been dis- 

 covered in any Tertiary deposit, or is known to 

 exist at the present day. 



Passing on next to the Vertebrate Animals of the 

 Cretaceous period, we find the Fishes represented 

 as before by the Ganoids and the Placoids, to which, 

 however, we can now add the first known examples 

 of the great group of the Bony Fishes or Teleosteans^ 

 comprising the great majority of existing forms. 

 The Ganoid fishes of the Cretaceous {Lepidottts, 

 Pycnodus, &c.) present no features of special in- 

 terest. Little, also, need be said about the Placoid 

 fishes of this period. As in the Jurassic deposits, 

 the remains of these consist partly of the teeth of 

 genuine Sharks {Lajnna, Odontaspis,8>LC.), and partly 

 of the teeth and defensive spines of Cestracionts, 

 such as the living Port-Jackson Shark. The pointed and sharp- 

 edged teeth of true Sharks are very abundant in some beds, such 

 as the Upper Greensand, and are beautifully preserved. The 

 teeth of some forms {Carcharias, &c.) attain occasionally a 

 length of three or four inches, and indicate the existence in the 

 Cretaceous seas of huge predaceous fishes, probably larger than 

 any existing Sharks. The remains of Cestracioftts consist 

 partly of the flattened teeth of genera such as Acrodus and 

 Ptychodus (the latter confined to rocks of this age), and partly 

 of the pointed teeth of Hybodus, a genus which dates from the 

 Trias. In this genus the teeth (fig. 206) consist of a principal 

 central cone, flanked by minor lateral cones; and the fin- 



Fig. 205.— 



Guard of 

 BelemniteUa 

 tnucronata. 

 White Chalk. 



Fig. 206.— Tooth 

 of Hybodus. 



Fig. 207. — Fin-spine oi Hybodus. Lower Greensand. 



spines (fig. 207) are longitudinally grooved, and carry a series 

 of small spines on their hinder or concave margin. Lastly, 



