CETACEA. 421 



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 present 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 founded 

 the genera Choneziphius and Belemnoziphius, both of which 

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

 also occurs in the Crag, but unlike the preceding it is repre- 

 sented by existing species. Besides the "beaks," some fossil 

 teeth have been found, which may perhaps be referable to mem- 

 bers of this family. 



Fam. 5. Zeugfodontida. The members of this family differ 

 from all existing Odontoceti in the possession of molar teeth im- 

 planted by two distinct fangs. The Zeuglodonts are entirely 

 extinct, and they are exclusively confined to the Eocene, Mio- 

 cene, and Pliocene periods. The chief genera are Zeuglodon 

 and Squalodon, 



Zeuglodon (fig. 349) is distinguished by its elongated snout, 

 conical incisors, and molar teeth with triangular serrated 

 crowns, implanted in the jaw by two roots. Each molar looks 



Fig. 340. Zeuglodon cetoides. A, Molar tooth, natural size ; B, Vertebra, reduced. 

 From the Middle Eocene of North America. (After Lyell.) 



as if it were composed of two separate teeth united on one side 

 by their crowns ; and it is this peculiarity which is expressed 

 by the generic name. The species of Zeuglodon are Eocene 

 and Miocene, one of the best known being the great Z. 



