XXV] SKULL 641 



bones. To the last-named a wedge shaped bone termed the vomer 

 is attached which projects downwards and divides the air-space 

 above the palatal folds into two. The name (Lat. vomer, plough- 

 share) is derived from the shape of the bone in Mammals; it is 

 inappropriate as a description of its shape in other Craniata. 

 Recent discoveries in the Theromorpha make it plain that the 

 Mammalian vomer is the representative oftheparasphenoid bone 

 of Amphibia and the lower Reptiles. The vomers of Amphibia and 

 Reptilia are represented in the Theromorpha by a pair of small 

 bones termed prevomers attached to the underside of the internasal 

 septum in front of the unpaired bone which corresponds to the 

 vomer of Mammalia. The pterygoid bones take the form of thin 

 vertical plates ; they are attached throughout their whole length to 

 the side wall of the cranium by the " alisphenoid." As in Croco- 

 dilia and desmognathous Birds the palatal folds are united in the 

 middle line; the bones supporting them are processes of the pre- 

 maxillary, maxillary and palatine bones. Between the pterygoid 

 bones however the palatal folds form a purely muscular bridge, 

 called the soft palate, which ends posteriorly in a projecting lobe, 

 called the uvula, lying close to the glottis. The processes of the 

 palatine bones always meet so as to form a bony bridge, called 

 the hard palate; those of the premaxilla and maxilla do so to 

 a certain extent, leaving however vacuities known as the anterior 

 palatine foramina (19, Fig. 315). (The posterior palatine 

 foramina are small holes in the palatine bones for the passage of 

 blood-vessels.) As in Chelonia, there is only a lower temporal 

 arcade, which is formed mainly by the cheek-bone or jugal. There 

 is however no quadratojugal, and the jugal joins a process of the 

 squamosal, which is a large bone covering the side of the skull 

 and almost concealing the conjoined bones of the auditory capsule 

 from view. It is characteristic of Mammalia that these bones, 

 which in the embryo are distinct from one another, unite to form a 

 single bone, called the peri-otic, which is fused to the squamosal. 

 In Reptiles, on the other hand, the epi-otic joins the supra-occipital 

 and the opisthotic the exoccipital, while the pro-otic remains 

 distinct. The outer ear, the funnel-shaped passage leading into 

 the tympanum, which is termed the meatus auditorius ex- 

 ternus, is surrounded by a bone called the tympanic, often 

 swollen into a rounded form and then termed the tympanic 

 bulla. There is often a tube -like prolongation of this bone into 

 the base of the ear-flap or pinna. 



s. & M. 41 



