104 MAMMALIAX DESCENT. [Lect. IY. 



the hind part of the floor is formed by a strong fascia of 

 membrane, and not by bone. This small kind gives us 

 the most backwardly placed " basi-pterygoid processes " 

 seen in any skull. The human anatomist will not 

 remember the term " basi-pterygoid process." It was 

 created a few years since for those spurs that grow" 

 out from the base of the skull to catcli the pterygoid 

 bones in Lizards and many Birds. 



In our solid cranio-facial architecture the bones of tlie 

 upper face have all been drawn up to the floor of the 

 skull, and have become largely confluent therewith, or 

 at least united by immovable sutures. Our internal 

 pterygoid plates form the inner pair of skull flanges, 

 right and left; the outer plates are the external ptery- 

 goids, and are mere outgrowths of the larger wings 

 of the sphenoid bone (alisphenoid). In a Sheep these 

 outer plates grow from the sphenoid, much nearer the 

 mid-line, and nearer still in the Guinea-pig ; in that type 

 the pterygoid bones, or inner j^lates, are carried on those 

 inwardly placed outer spurs — as in Lizards and Birds. 

 All these things are specialisations, very exquisite, but 

 not belonging to the deep things of morphology. In 

 the Ant-eater it is the basal part of the occipital ring 

 which gives off these spurs. 



The contrast in length between the skull of the great 

 Ant-eater and that of the Sloth is very marked indeed ; 

 one is the longest in the Class, and the other is almost 

 the shortest. 



