90 DAVIDSON BLACK 



mechanism due primarily to the monismostylic type of skull 

 characteristic of these forms. 



In the second group, as exemplified by Boa, Varanus, and also 

 Anolis (Willard, 1. c), the motor V nucleus is for the most part 

 ventrally or rather peripherally placed, and in relative size it is 

 noticeably larger than the corresponding nucleus in Alligator 

 and chelonians (figs. 11 and 12). In the former animals the 

 chief bulk of the motor V nucleus lies in a position analogous to 

 that occupied only by the caudoventral moiety in Alligator. 

 In Varanus and Anolis a small portion of this nucleus still retains 

 a relatively dorsal position, but in Boa this dorsal moiety has 

 been lost. 



The ventral (peripheral) position of the motor V nucleus in 

 the above-mentioned representative of the reptilian group 

 Squamata is evidently to be correlated with the increased com- 

 plexity of the V jaw musculature. The degree of differentiation 

 of the m. capiti-mandibularis in these forms, however, presents 

 no advance over the condition obtaining in this muscle in Alli- 

 gator ; but in the development of the so-called pterygoid muscular 

 complex a marked specialization is evident. In the development 

 of the streptostylic form of skull the complexity of this muscula- 

 ture is directly proportionate to the degree of movability of the 

 bones concerned and the pterygoid muscular complex of the 

 Squamata has been acquired to meet the needs of the movable 

 quadrate, and thus, as Adams has emphasized, must be a caen- 

 otelic character. 



If, then, the peripheral position and large size of the motor 

 V nucleus in the Squamata examined be correlated with the 

 increased complexity of the V jaw musculature, it follows from 

 the above remarks that it must be the pterygoid part of this 

 musculature that has exerted most of this influence. The ventro- 

 peripheral position of the V motor nucleus in these forms must 

 also be regarded as a caenotelic character independently acquired. 



As Kappers has pointed out, the most important center acting 

 reflexly upon the motor V nucleus in reptiles appears to be that 

 of the chief terminal nucleus of its own sensory root (31). To 

 this general statement no further facts can be added at this 

 time. 



