THE CRETACEOUS PERIOD. 



283 



number of other allied forms, which cannot be noticed here. 

 Not a single one of these genera, further, has hitherto been 

 detected in any strata higher than the Cretaceous. We may 

 therefore consider that these wonderful, varied, and elaborate 

 forms of Ammonitidce constitute one of the most conspicuous 

 features in the life of the Chalk period. 



The Dibranchiate Cephalopods are represented partly by the 

 beak-like jaws of unknown species of Cuttle-fishes and partly 

 by the internal skeletons of Belemnites. Amongst the latter, 

 the genus Belemnites itself holds its place in the lower part of 

 the Cretaceous series; but it disappears in the upper portion 

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

 Belcmnitella (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 Belemnitidce 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 (Lepidotus, 

 Fycnodus, 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 (Lamna,Odontaspis, &c.),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 Cestracionts 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 



rig. 205. 

 ueUa mu- 



