464 THE VERTEBRATE SKELETON. 



cartilaginous process, and is fused behind with the presphenoid 

 (fig. 93, 8). The basi-occipital (fig. 93, 1) too is fused with 

 the basisphenoid. The foramen rotundum is confluent with 

 the sphenoidal fissure, and the foramen ovale with the foramen 

 lacerum medium and the foramen lacerum posterius. The 

 palate is mainly formed by the maxillae ; the premaxillae and 

 palatines (fig. 93, 17), though both meet in symphyses, forming 

 very little of it. The pterygoids vary in size in the different 

 genera, sometimes as in Lagenorhynchus and Delphinus meet- 

 ing in the middle line, sometimes as in Phocaena and Globi- 

 cephalus (fig. 93, 18) being widely separated. The tympanic 

 and periotic are not fused together, and the periotic has 

 generally no bony union with the rest of the skull. The 

 mandible is rather slightly developed, with the rami straight, 

 compressed and tapering to the anterior end. The condyle is 

 not raised at all above the edge of the ramus ; the angle is 

 rounded and the coronoid process is very small. Platanista has 

 a curiously modified skull ; the rostrum and mandible are 

 exceedingly long and narrow, and arising from the maxillae 

 are two great plates of bone which nearly meet above. 



In the Physeteridae the skull is raised into a very promi- 

 nent crest at the vertex behind the nares. In front of this 

 in Hyperoodon a pair of ridges occur, formed by outgrowths 

 from the maxillae. In the old male these ridges reach an 

 enormous size and almost meet in the middle line. In Phy 

 seter, the Sperm whale, these ridges are not developed ; the 

 maxillae and premaxillae unite with the other bones of the 

 crest enclosing an enormous half basin-shaped cavity, at the 

 base of which are the very asymmetrical anterior narial aper- 

 tures. 



In all living Cetacea the hyoid has the same general shape, 

 consisting firstly of a crescentic bone formed by the fusion 

 of the thyro-hyals with the basi-hyal, and secondly of the 

 anterior cornu formed principally by the strong stylo-hyal. 



UNGULATA. None of the distinctive characters separating 



