THE SKULLS OF MAMMALIA. 269 



process is even more largely developed, to contain the incisor 



tusks of the animal. 



The maxillae, also large and prolonged forwards, have very 

 thick and long palatine processes, separated by a wide incisive 

 foramen from the premaxilla. The palatine process of the 

 palatine is also very thick, but it is shorter than deep, so that 

 the posterior nares, which open behind it, are placed vertically 

 under the hinder part of the anterior nares, in the dry skull. 

 The vomer, thick and stout behind, thin and ridge-like in 

 front and above, embraces the lower edge of the ethmoid, and 

 is suturally united to both the palatines and the maxillaries. 



The skulls of the Sirenia have resemblances on the one side 

 with those of the ungulate Mammals and Proboscidea ; on the 

 other, with those of the Cetaeea, but yet differ in many and 

 most important respects from all. 



. The skulls of the Cetaeea present more singular modifications 

 than those of any other Mammalia. In all these animals, the basi- 

 cranial axis is concave superiorly, and the primitive separation 

 between the basi-sphenoid and presphenoid persists for a long time. 



The vomer is very long, and extends backwards on the base 

 of the skull at least as far as the basi-sphenoid, and sometimes 

 covers the whole length of that bone. 



The ethmoid has its posterior edge perpendicular, or nearly 

 so, to the basi -cranial axis, and the foramina for the exit of the 

 olfactory nerve are small or obliterated. 



The frontals enter but very little into the roof of the skull, 

 largely into its anterior and lateral walls. They are prolonged 

 outwards and forwards into the long and broad supra-orbital 

 processes, which are concave interiorly, where they form the 

 roof of the orbital cavity. 



The parietals hardly appear at all, externally, upon the top 

 of the skull, their median parts being obscured or interrupted 

 by the inter-parietal and supra-occipital. They occupy a large 

 space, however, in the temporal fossae. 



The ex-occipitals and supra-occipitals are enormous. The 

 latter, usually increased by coalescence with the large inter- 

 parietal, extend up to, or beyond, the vertex to meet the 



