xii.] CETACEA. 193 



Between the stylohyal and the basihyal are one or two dis- 

 tinct short cartilages articulated together by synovial joints, 

 one of which occasionally becomes ossified. 



In many of the Delphinidce the rostral portion of the skull 



proportionally more elongated and compressed than in 



le species just described ; notably so in Pontoporia, a South 



.merican genus. In this animal the mandible has a very 



>ng symphysial portion, the two rami being parallel and 



mkylosed for more than half their length, and diverging 



>nly in the posterior portion. 



The Sousou, or Platafiista, a Dolphin inhabiting the 



■ivers of South Asia, has also a remarkably elongated and 



>mpressed rostrum and mandible, and the cranial portion 



>f the skull presents several structural peculiarities. The 



)rbit is extremely small, the temporal fossa large, and 



le zygomatic processes of the squamosal are greatly 



jveloped. From the outer edge of the ascending plates 



>f the maxillae, which lie over the frontals, great crests of 



>ne, smooth externally, but reticulated and laminated on 



leir inner surface, rise upwards, and, curving inwards, nearly 



leet in the middle line, above the upper part of the face. 



The Physeteridce, including the genera Zifihius, Hy- 

 vodon, Physeter, and their allies, present several special 

 odifications of the skull. The bones of the face and 

 cranium, meeting at the vertex, are raised so as to form a 

 more or less elevated transverse prominence or crest behind 

 the anterior nares, generally curved forwards at its upper 

 edge. The bone which corresponds to the malar in other 



iolphins is usually divided into two, one of which may 

 present the lachrymal. The pterygoid bones are thick, 

 oduced backwards, meeting in the middle line for a 

 ►nsiderable space, concave on their outer side, but not 



