xiir 



PHYLUM CHORDATA 



535 



The shull (Fig. 1152) is characterised by its extreme hardness. 

 The cranial cavity is rather long and narrow as compared with 

 that of the Cetacea. Although the supraoccipital (SO?) is pro- 

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

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

 The frontals develop broad supra-orbital plates. Tlie zygoma 

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

 they are relatively further forwards. The nasals are rudimentary. 



ExO 



Flii. 1152.— Section of skull of IMTanatee (Manatus senegalcnsis). Letters as in p'ig. ll.JO. In 

 addition, ET. etlimo-turbinal ; T>j. tympanic. (After Flower.) 



The tympanic and periotic are readily separable from the other 

 bones. There are enormous premaxillas in the Dugong. The 

 mandible has a well-developed ascending ramus and coronoid 

 process (qh). 



The scwpvla 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 sj^ine is situated about 

 the middle ; the acromion is directed downwards. The coracoid 

 is fairly large, 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 



