282 



DR. H. GADOW ON EVOLUTION 



[Mar. 20, 



This question must be left in abeyance. As a rule, however, the 

 larger species seem to have more numerous scales upon the thighs 

 and elewhere than their nearest smaller relations. This may be 

 in the nature of things ; it is quite possible that these many-jointed 

 armourings cease to fulfil their purpose when the individual com- 

 ponents pass beyond a certain size. 



The presence or absence of a separate frenocular plate, so often 

 relied upon in systematic works, is quite unreliable. Its absence 

 is due equally often to suppression as it is to fusion with some 

 neighbouring plate. 



Text-fig. 63. 



Lepidosis of the under surface of the forearm. 



A=C mexicanus, Balsas No. 3. Right forearm. Several rows of enlarged 



polygones clown to wrist. 

 B=C mexicanus, Balsas No. 2. Right forearm. 

 C=C. mexicanus, Cuernavaca No. 8. Left forearm. Large scutes. 

 D=C. mexicanus, Cuernavaca No. 10. Left forearm. Large scutes. 



It seems reasonable to assume that 4 sujjraoculars lepresent 

 the more primitive condition, whence, by reduction of either the 

 anterior or the posterior scute, the number is reduced to 3. In 

 most of the text-figs. 61-83 these features are clearly visible. 



The composition of the collar and the protection of the posterior 

 side of the forearm are difficult to describe in a few words. To 

 avoid the drawback of vague terms, the reader is referred to a 

 series of illustrations which are intended to standardise the 

 phraseology employed in this paper (text-figs. 62, 63, 64, 65). 



Some systematists have laid stress upon other characters. 

 Peters, for instance, found that the first upper labial was denti- 

 culated in C. dejij^ei ; this is best seen on the inside, but it is by 

 no means always the case in that species, while it occurs also, 

 occasionally, in C. immutabilis, in C. communis from Cozumel 

 Island, and perhaps in othei'S. 



Unpracticable were also the, at first sight, great differences 

 whether the keel of the tail- scales runs parallel, subparallel, or 



