CETACEA. 273 



terrestrial mammal. The nasal bones are large square elements articu- 

 lated by a broad base with the frontals, projecting over the nasal chamlxjr, 

 and abruptly truncated in front. The rostrum is extremely slender. The 

 syiuphysis of the mandible is somewhat more than half the length of the 

 jaw, and it terminates in front in an upwardly-directed, toothless, spatu- 

 late end, quite unlike that of any existing Odontocete. The teeth are 

 unknown, but they seem to have been numerous and slender, and there 

 are about 25 sockets in each ramus of the mandible. A few cervical 

 vertebrae found with the skull are not fused together, and are moderately 

 long, like those of the modern Platauistidse, which are generally regarded 

 as the most primitive of existing Cetaceans. 



Physodon. This name was originally given to some enamel-capped 

 teeth, evidently of early sperm-whales, from the Miocene and Pliocene of 

 Europe. The skulj from the Patagonian Formation of Chubut, with 

 .similar teeth, can thus only be placed provisionally under the same generic 

 denomination. Teeth of such a form may have belonged to several genera. 

 In any case, the South American Miocene skull described under the name 

 of Physodon patagonicus, measures only about a metre and a half in 

 length, and shows very clearly its relationship to the modern sperm-whales 

 (Physeteridse) by having the large deep cavity in the fronto-parietal region 

 for the reception of the accumulated spermaceti. The nasal bones are 

 unknown. The premaxillse extend some distance in advance of the maxillae, 

 forming about a third of the rostrum, and bearing three pairs of the upper 

 teeth. All the teeth are simple cones with an enamel cap, and they seem 

 to have been 22 in number on each side of the upper jaw, 24 in each 

 ramus of the mandible. 



Before the end of the Pliocene period all the most specialized 

 types of Odontoceti seem to have been evolved. The strange 

 compact rostra of the Ziphioid genera, such as Mesoplodon, are 

 even common fossils in the Pliocene of England and Belgium. 

 Fragments of the tusk and rostrum of a narwhal (Monodon) are 

 also known from the Cromer Forest Bed (base of Pleistocene). 



Sub-Order 3. Mystacoceti. 



The whalebone whales, or MYSTACOCETI, must have been 

 derived from toothed ancestors, as is indicated by the presence 

 of calcified rudiments of teeth in the foetus of more than one 

 existing genus. Apart, however, from the development of 

 baleen and the remarkable form and proportions of the mouth, 

 they are much less specialized than any of the surviving Odon- 

 toceti. They must therefore have diverged from the latter 

 before these lost their olfactory organ and many other primitive 

 w. 18 



