582 CALIFORNIA ACADEMY OF SCIENCES. 



are among the most constant: The bones of the skull 

 grow thicker and in very large specimens become spongy. 

 The processes of the mesethmoid become depressed; and 

 the ventral process of the basisphenoid, when present at 

 all, sometimes suffers complete, and always partial ab- 

 sorption. The interorbital space grows relatively wider, 

 this being one of the most striking and constant varia- 

 tions. In the present paper the width of this space is 

 always given as measured at its narrowest part (which usu- 

 ally falls immediately behind the preocular spines), and 

 compared with the total length of the base of the skull. 

 In a young vexillaris, th^ ratio of interorbital width into 

 the length of the base of the skull is 5^, in a medium- 

 sized one 4I, and in a large one 4. In a young maligej' 

 it is 4I, in an old one 4^ ; in a young niiniatits 3^, in an 

 old one 3tt; i^^ 2i young Jf,avidiis 3^^, in an old one 3. 

 In a very young ruberrimiis it is 6f , in one two or three 

 times as large 54, in one in which the cranial ridges are 

 almost competely serrated 5, and in a ver}^ large, old 

 specimen 43^3. 



The degree of approximation of the parietals seemed 

 at first to be a valuable character, and it will be seen from 

 the key given below that in several parts of the group 

 closely related species have the parietals in contact; but 

 while it serves well as a character of subordinate import- 

 ance, the mere fact that any two species have parietals 

 which meet or overlap is no proof of affinity unless it is 

 supported by other agreements. 



The most reliable cranial characters for the purpose of 

 classification of the species are: the degree of curvature 

 of the base of the skull; the convexity or concavity of the 

 interorbital space and its relative width; the direction of 

 the mesethmoid processes ; the degree of development of 

 the ventral process of the basisphenoid; and the strength 



