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-allied genus 
Belemnitella (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 Aelemnitide has hitherto been dis- 
covered in any Tertiary deposit, or is known to 
exist at the present day. 
Passing on next to the Vertebrate Animats of the 
Cretaceous period, we find the /zshes 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 Gony Fishes or Teleosteans, 
comprising the great majority of existing forms. 
The Ganoid fishes of the Cretaceous (Lepidotus, 
Pycnodus, &ec.) present no features of special in- 
terest. Little, also, need be said about the Placoid | 
fishes of this period. As in the Jurassic deposits, — Fig. 205.— 
: : Guard of 
the remains of these consist partly of the teeth of  Becemnitella 
genuine Sharks (Lamna, Odontaspis, &c.), and partly § Yyregea. 
of the teeth and defensive spines of Cestracionts, é 
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 ydodus, 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. 206.—Tooth Fig. 207.—Fin-spine of Hybodus. Lower Greensand. 
of Hybodus. 
spines (fig. 207) are longitudinally grooved, and carry a series 
of small spines on their hinder or concave margin. Lastly, 
