ORDER CETACEA. 



1307 



distinguished by the denticulations being more developed on the 

 posterior than on the anterior border. The so-called Rhizoprion is 

 merely a species of this genus. 



Family Delphinid^e. — In this, the last family of the Cetacea, 

 the teeth are simple, and usually numerous in both jaws ; and the 

 length of the mandibular symphysis may be very small, and never 

 exceeds one-third of that of the entire ramus. The periotics (fig. 

 1 1 79), which are frequently found in a fossil condition, are 

 readily distinguished from those of the Physeteridce, by having the 

 anterior facet which articulates with the tympanic marked by a 



78. — The common Dolphin (Delphinus delphis). Reduced. 





number of grooves, as well as by the narrowness of the posterior 

 half of their tympanic aspect. This family comprises all the Ceta- 

 ceans commonly known as Porpoises, Grampuses, Killers, and Dol- 

 phins. Remains of the Narwhal (Monodon monoceros) are found 

 in the Norfolk Forest-bed and the Pleistocene of Alaska. The 

 genus Delphinapterus, now represented by the "White Whale," 

 occurs in the Lower Pliocene of Tuscany, as well as in the Pliocene 

 and Miocene of other parts of Europe. An extinct species of 

 " Killer- Whale " (Orca) is found in both the Italian and English 

 Pliocene. The existing Globicephalus 

 me/as, or " Black-fish," has left its re- 

 mains in the superficial deposits of 

 Essex ; while the extinct G. uncidens 

 occurs in the Suffolk Crags. A left 

 periotic of the latter species is repre- 

 sented in fig. 1 1 79, in order to show 

 the features characteristic of this family. 

 The existing Pseudorca crassidens was 

 originally described from a subfossil 

 cranium found in the fens of Lincoln- 

 shire. Of the smaller Dolphins it is probable that the genera 

 Tursiops, Lagenorhynchus, and Delphinus (as now restricted) are all 

 represented in the Pliocene of Europe. Eurhinodelphis is a long- 

 snouted genus from the Pliocene of Belgium and Italy, which is 



Fig. 1 179. — The left periotic of 

 Globicephalus uncidens ; from the 

 Coralline Crag of Suffolk. 





