[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 Salmon family (Salmonidoe), the Herring family (Clupeidoe), and the Perch family (Percidoe).  All these fishes have thin, horny, overlapping scales, symmetrical ("homocercal”) tails, and bony skeletons.  The genus Beryx (fig. 208, 1) is one represented by existing species at the present day, and belongs to the Perch family.  The genus Osmeroides, again (fig. 208, 2), is supposed to be related to the living Smelts (Osmerus), and, therefore, to belong to the Salmon tribe.

[Illustration:  Fig. 205.—­Guard of Belemnitella mucronata.  White Chalk.]