xii.] CETACEA. 217 



to a small ossified tympanohyal, which becomes ankylosed to 

 the periotic in the usual situation, close to the stylomastoid 

 foramen ; it has also a strong ligamentous attachment to 

 the prominent rough paroccipital process of the exoccipital. 

 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 Delphinidcz the rostral portion of the skull 

 is proportionately more elongated and compressed than in 

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

 American genus. In this animal the mandible has a very 

 long symphysial portion, the two rami being parallel and 

 ankylosed for more than half their length, and diverging 

 only in the posterior portion. 



The Sousou, or Platanista, a Dolphin inhabiting the 

 rivers of South Asia, has also a remarkably elongated and 

 compressed rostrum and mandible, and the cranial portion 

 of the skull presents several structural peculiarities. The 

 orbit is extremely small, the temporal fossa large, and 

 the zygomatic processes of the squamosal are greatly 

 developed. From the outer edge of the ascending plates 

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

 bone, smooth externally, but reticulated and laminated on 

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

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



The PhyseteridcR, including the genera Ziphins, Hy- 

 peroodon, Physeter, and their allies, present several special 

 modifications 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 



