SCHl'LTK, SKI \VHALE. 477 



this view it is necessary to enter into details. The external pterygoid, the ectal surface of 

 which has been described, is not a thin plate but a massive bone of irregular pyramidal shape. 

 By its summit entally it joins the as yet cartilaginous processus alaris, beyond which it is con- 

 tinuous with the base of the processus ascendens. It thus conforms literally to the definition 

 of the external pterygoid, being a descending process of the alisphenoid and ossifying from a 

 centre common to it and the processus ascendens. Notwithstanding this typical conformation 

 in the basal region, we are confronted ectally in the temporal fossa with a suture between the 

 ala temporalis and the external pterygoid, and it is this highly aberrant and peculiar character 

 which requires explanation. In less modified skulls these elements ectally are widely separated, 

 a portion of the zygomatic fossa intervening between them. Here it appears that incident to 

 the expansion of the external pterygoid to a pyramidal mass, it has come to be secondarily 

 appressed to the ala temporalis (alisphenoid) with concomitant reduction of the zygomatic 

 fossa, the highly significant remnant of which, and index of the whole process, persists as the 

 suture between the external pterygoid and alisphenoid. 



The caudal surface of the external pterygoid is exposed on removal of the otic capsule. Dor- 

 sad it extends to the processus alaris, ventrad to the base of the hamular process, from which it 

 is separated by a suture. Laterad the bone is drawn out into a stout process, which lying mesal 

 to the foramen ovale, is superficially overlain by the squamosal, so that the ectal orifice of the 

 foramen lies in the suture between that bone and the external pterygoid. The apex of this pro- 

 cess touches and is very firmly united by fibrous tissue to the tegmen tympani. Mesad the 

 external pterygoid extends to the nasal fossa and here intervenes between the hamular and vagi- 

 nal processes. These are both of large size, and immediately rostral to them the external ptery- 

 goid comes into the lateral wall of the nasal fossa, to a degree usurping the place of the internal 

 pterygoid plate and taking part in the formation of the choanse. The caudal surface of the 

 external pterygoid is crossed by a transverse ridge extending from the lateral process to its 

 mesal margin opposite the vaginal process. 1 Dorsal to this ridge the surface is concave and in 

 apposition with the first turn of the cochlea. Ventrally the surface is also concave and lodges 

 the Eustachian tube, giving origin also to the tensor tympani. For this reason it seems that 

 the concavity bounded by this surface laterad and rostrad, and mesally by the vaginal process, 

 should be considered scaphoid and not pterygoid fossa. 



Beauregarde, in his description of this region in a young specimen of B. rostrata ( = acuto- 

 rostrata), records essentially similar conditions. The fossa in the pterygoid is ovoid, it is limited 

 "en dedans par un crete eleV6e fournie par le pterygo'ide, en avant par une apophyse digitiforme 

 de pterygo'ide que fait saillie en dedans, au dessous de la crete susdite et qui limite entre elle et 

 cette crete un espace dans lequel passe la trompe d'Eustache." The last fact and his excellent 

 illustration makes it clear that the structures in question are the vaginal and hamular processes 

 of the internal pterygoid plate. 



The internal pterygoid lies in general rostrad of the external, in which relative position it 

 appears both on the ectal surface of the skull and in the nasal fossa. Ectally it overrides the 

 external pterygoid by its caudal margin, while its vaginal process is prolonged on the mesal, 

 its hamular process on the ventral aspect of the latter bone. Thus the external pterygoid is 

 mortised into the internal from behind. 



The squamosal in ventral view has a triradiate form; the stout zygomatic process projects 



1 Cf . Beauregarde, H. Recherches sur 1'appareil auditif chez les mammif6res. Jour, de 1'Anat. et de la Phys., An. XXIX. 



