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 _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 _Belemnitidoe_ has hitherto been discovered in any Tertiary deposit, or is known to exist at the present day. [Illustration: Fig. 203.--_Turrilites catenatus_. The lower figure represents the entire shell; the upper figure represents the base of the shell seen from below. Gault.] [Illustration: Fig. 204.--a, _Ptychoceras Emericianum_, reduced--Lower Greensand; b, _Baculites anceps_, reduced--Chalk; c, Portion of the same, showing the folded edges of the septa; d, _Crioceras cristatum_, reduced--Gault; e, _Scaphites oequalis_, natural size--Chalk; f, _Hamites rotundus_, restored--Gault.] 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, Pycnodus_, &c.) present no features of special interest. 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 central cone, flanked by minor lateral cones; and the fin-spines (fig. 207) are longitudinally grooved, and carry a series of small spines on their hinder or concave margin. Lastly, the great modern order of the Bony Fishes or _Teleosteans_ makes its first appearance in the Upper Cretaceous rocks, where it is represented by forms belonging to no less than three existing groups--namely, the