SIRENIA AND CETACEA. 



315 



siou of a pointed snout (the " beak " or " rostrum "), single 

 blow-hole, and small dorsal fin ; and by their dentition. 

 The upper jaw is edentulous, any teeth which may be pre- 

 sent not cutting the gum. The lower jaw, on the other 

 hand, possesses usually a single pair of teeth, which are 

 sometimes tusk-like, but which in other cases are concealed 

 by the gum. 



The rostrum of these Cetaceans is of great density, and 

 has often been preserved in a fossil state, usually presenting 

 itself as a bony cylinder or elongated cone, generally more 

 or less water-worn. Upon fossils of this nature have been 



Fig. 621. — The common Dolpliiii {Deiphimis delphis). 



founded the genera ChonezijjJims and BelemnQzi2Jhius, both of 

 which occur in the so-called " Crags " (Pliocene). The genus 

 Ziplims also occurs in the Crag, but unlike the preceding- 

 it is represented by existing species. Besides the " beaks," 

 some fossil teeth have been found, which may perhaps be 

 referable to members of this family. 



Fam. 5. Zeuglodonticlm. — The members of this family 

 differ from- all existing Odontoceti in the possession of molar 

 teeth implanted by two distinct fangs. Incisor teeth are 

 likewise present, and the animal is diphyodont. The Zeuglo- 

 donts are entirely extinct, and they are exclusively confined 

 to the Eocene, Miocene, and Pliocene periods. The chief 

 genera are Zeuglodon and Squalodon. 



Zeuglodon (fig. 622) is distinguished by its elongated 

 snout, conical incisors, and molar teeth with triangular ser- 

 rated crowns, implanted in the jaw by two roots. Each 

 molar looks as if it were composed of two separate teeth 

 united on one side by their crowns ; and it is this peculiarity 



