LEPUS 135 



contain the testes. This ventral position of the testes is a new 

 feature, peculiar to most adult mammals. 



Skull.— The skull has two occipital condyles, formed from 

 the exoccipitals. The floor is formed by basioccipital, basi- 

 sphenoid, presphenoid, and vomer, the latter representing the 

 anterior portion of the parasphenoid of lower forms. The 

 mesethmoid is perforated by a number of pores through which 

 branches of the olfactory nerve run to the nasal sacs ; it is 

 known as the cribriform plate. Anteriorly, the mesethmoid 

 extends as the vertical septum nasi, which separates the 

 cavities of the nasal capsules. 



The roof of the skull is formed by the supraoccipital, 

 parietals, frontals, and nasals. The bones of the auditory 

 capsules are fused to form the periotics, which form the hinder 

 part of the side of the brain-case. The remainder of the side 

 is formed by the squamosals, alisphenoids (corresponding to 

 the epipterygoids of reptiles), and orbitosphenoids. There is 

 a small lachrymal bone near the front of the orbit. 



The margin to the upper jaw is formed by the premaxillae 

 and the maxillae. From the maxillae the jugals extend back- 

 wards and meet a process (zygomatic) of the squamosal forming 

 (with the jugal) the " cheek bone." The roof of the mouth 

 is a false palate, formed by flat extensions of the maxillae and 

 palatine bones meeting their fellows of the opposite side 

 ventral to the true roof of the mouth, and enclosing the nasal 

 passage. The pterygoids are small bones behind the palatines, 

 and at the sides of the basisphenoid. The vomer representing 

 the parasphenoid is covered over by the false palate. Large 

 tympanic bullae lie beneath the periotic and contain the 

 tympanic cavity. The nasal cavities contain a number of 

 scroll- like turbinal bones (see Figs. 143, 149, and 150). 



The wall of the skull is pierced by a number of holes or 

 foramina through which nerves and blood-vessels pass out 

 and enter. These foramina are commonly situated between 

 different bones, for the nerve or blood-vessel developed first 

 and the bones formed afterwards. At first these bones are 

 small, but as they grow they meet one another forming sutures, 

 and foramina are open sutures. Occasionally the bone grows 



