XIII 



PHYLUM CHORDATA 



503 



The skull (Fig. 1083) is characterised by its extreme hardness. 

 The cranial cavity is rather long and narrow as compared with 

 that of the Cetacea. Although the supra- occipital (0.). is P ro ~ 

 duced forwards on the upper surface of the skull for a considerable 

 distance, it does not separate the parietals (Pa.) from one another. 

 The frontal develops broad supra-orbital plates. The zygoma 

 is stout. As in the Cetacea the external nares are very wide, 

 but are relatively further forwards. The nasals are rudimentary. 

 The tympanic and periotic are readily separable from the other 



MX. 



PMx 



ExQ 



FIG 1083 Section of skull of Manatee (Manatus senega! ensis). Letters as in Fig. 1081. In 

 addition, ET. ethmo-turbiiial ; Ty. tympanic. (After Flower.) 



bones. There are enormous pre-maxillse in the Dugongs. The 

 mandible has a well-developed ascending ramus and coronoid 



process (cp.}. 



The scapula of the Sirenia is much more like that of the terrestrial 

 Mammals than is that of Cetacea, but is nearer that of the Seals ; 

 it is narrow and curved backwards. The spine is situated about 

 the middle: the acromioii is directed downwards. The coracoid 

 is fairly well developed, and of a conical shape. The clavicle is 

 absent, as in the Cetacea. The skeleton of the arm also departs 

 less from the ordinary Mammalian type than in the Cetacea. The 



