374 Universiiij of CoJifoniia PnliJicaiions in Zoohxjn [Vol. 21 



in-ement than the pahitihir U^nfith or the palatal lenofth, which are so 

 often used in diagnosing: various species of mammals, would be from 

 the anterior tip of the premaxillae to the maxillo-palatine suture. 



Pterygoids. — The pterygoids are not extensive at any stage of 

 development and contribute to the skull only the processes bearing 

 that name. In ^i, as shown in figure 20, each pterygoid is ossified 

 and is in contact with a palatine. Its changes in shape and in relative 

 size are shown in ventral view in figures 15-20. It is attached to 

 the posterior end of the pterygo-alisphenoid ridge and in A, as may 

 be seen from figure 20, only to the extreme posterior end of this 

 ridge. In A to D the pterygoid is closely appressed to the underlying 

 bones. In E it is slightly raised. In G the pterygoid process lies on 

 a plane passing across the ventral surfaces of the tympanic bullae. 

 In Q each process projects 1 mm. or more below this plane. At this 

 time the anterior border of the pterygoid process projects downward 

 from the floor of the palate at an angle of 35 degrees. In A to i? the 

 shape is profoundly modified by the tympanic, which crowds in upon 

 the pterygoid and excavates its lateral border giving it the gibbous 

 shape shown in figures 17-20. In the pig, according to Parker (187-1, 

 p. 322), the pterygoid dents the tympanic, instead of the tympanic 

 denting the pterygoid, as is the case in Otospermophilus. In / and 

 in later stages, a tendency to straighten this curved border is manifest 

 but even in Q the process is curved outward. It would seem, how- 

 ever, that the retention of this curve is due to causes other than the 

 early influence of the tym])anic. This opinion is supported by the 

 fact that a rotation of 90 degrees occurs during the time that the 

 process is lifted from its horizontal position to that of an inclination 

 of 35 degrees whereby the border that was lateral in A becomes ventral 

 (skull with palate facing downward) in Q ; thus any effects that might 

 remain in Q from pressure exerted by the tympanic in earlier stages 

 would be present on the ventral border and not on the lateral face 

 of the process. That this process does rotate is indicated by the fact 

 that the palatal face of the pterygoid remains smooth whereas the 

 lateral face is modified at its base by the formation of considerable 

 osseous tissue, at precisely the proper point to cause rotation. The 

 deposition of additional bony matter at this point and the ultimate 

 shape of the hamular process are probably determined to a greater 

 degree by the effects of muscular development than by the influence 

 of the surrounding bones. The pterygoid processes fuse with the 

 basisplienoid in G. On the median surfaces of the pterygoids the 



