248 



ON THE STRUCTURE OF THE SKULL. 



In the roof of the skull (Fig. 98) a large inter-parietal, SO 1 , 

 which corresponds with the upper part of the squama occipitis 

 of Man, occupies an interval left, posteriorly, between the two 



Fig. 98. 



Fie. 98. — Side view of the skull of a Beaver. Ty, tympanic bone ; M, pars mastoidea ; 

 Pm, the downward process of the ex-occipital, called " paramastoid." 



parietals ; otherwise, the bones correspond with those found in the 

 roof of the skull of the Bird. The ethmoid, the vomer, the nasal 

 bones, the prernaxillae, maxillse, lachrymal, jugal, and squamosal 

 bones, the palatines, and the pterygoids of the Boclent, present 

 no difficulties to the student acquainted with the structure of the 

 Bird's and Keptile's skull ; but he will miss the pre-frontals, the 

 post-frontals, the quadrato-jugal, the transverse, and the quad- 

 rate bones, together with all the pieces of the lower jaw, save 

 the dentary. 



The post-frontals, the quadrato-jugal, the transverse, and 

 four out of the five missing pieces of each ramus of the lower jaw, 

 appear to have no representatives in the Mammalian skull. 



The pre-frontals, on the other hand, are represented by the 

 so-called " lateral masses of the ethmoid," with their develop- 

 ments, the superior and middle ethmoidal turbinal bones, wdiich 

 answer precisely to those of Man. A third turbinal, developed 

 from the primitive cartilaginous wall of the olfactory chamber, 

 eventually becomes united with the maxilla, and answers to the 

 inferior or maxillary turbinal of Man ; while, in the Beaver, there 

 is a fourth turbinal, connected with the superior turbinal and with 

 the nasal bones, which may be termed the " nasal turbinal." 



How far these well-defined turbinals of the Mammal answer 

 to the cartilaginous and osseous turbinals of Birds and Keptiles, 

 is a question w T hich has yet to be elucidated. 



