THE SKULLS OF MAMMALIA. *265 



On the other hand, some Ruminants carry to an extreme the 

 development of the frontal into a great supra-orbital arch, its 

 extension backwards in the middle line, and the concomitant 

 expansion of the supra-occipital forwards ; so that the parietals 

 of the Ox, for example, are reduced to a comparatively narrow 

 band in the middle line, while they expand widely in the 

 temporal fossae (Fig. 104). 



The crania of the purely aquatic Mammals, such as the 

 typical Seals, the Sirenia and the Cetacea, exhibit a certain 

 similarity of character in the midst of very wide and important 

 differences. 



The basi-cranial axis is either flat or slightly curved upwards 

 at its anterior and posterior extremities. The olfactory and 

 occipital planes are vertical, or nearly so. The squama occijritis, 

 alone, or united with large inter-parietal elements, extends upon 

 the vertex of the skull between the parietals, and approaches, or 

 even reaches, the frontals, so that the parietals are very much 

 shorter antero-posteriorly than at the sides and below. 



The frontals take but a small share in the formation of the 

 roof of the cranial cavity ; the nasals are relatively short, the 

 anterior nasal aperture relatively large, and the posterior often 

 situated far forwards. The prefontals, or lateral masses of the 

 ethmoid, are small or rudimentary. The tympanic and periotic 

 are always anchylosed together, and, whether connected or not 

 with the squamosal, are more easily detachable from the skull 

 than usual. 



The Seals are extreme aquatic modifications of the carni- 

 vorous type of cranial structure ; the Sirenia, of the ungulate 

 type. The Cetacea present resemblances to both. 



In the common Seal (PJwea vitulina) (Fig. 105) the cranial 

 cavity is exceedingly broad and spacious, and the cerebral 

 extends far further back over the cerebellar chamber, and is 

 much larger in proportion to it, than is usual in Carnivora. 

 There is a strong bony tentorium, and an osseous falx is more 

 or less developed. The basi-cranial axis, very thin and broad, is 

 curved, so as to be concave from before backwards. The syn- 

 chondrosis between the presphenoid and basi-sphenoid persists. 



