15 GENUS SEBASTODES. 583 



or weakness of the cranial ridges. These characters are 

 closely correlated, and furnish the only basis for the ar- 

 rangement of the species within the genus. In the j>auci- 

 spinis, ntc/anaps and -pinniger groups (see classification 

 below) the base of the skull is strikingly curved; the in- 

 terorbital space is always convex (at most flat, never con- 

 cave) and relatively wide, its width never being more than 

 3*4 in the length of the base of the skull; the mesethmoid 

 processes are never directed upward ; the ventral process 

 of the basisphenoid is absent, or reduced to a mere point 

 or at most occasionally present in very young specimens; 

 the cranial ridges are poorly or not at all developed and 

 the spines are delicate or absent. In the rosaceus-nebu- 

 /usits groups the base of the skull is straight or nearly so; 

 the interorbital space is always concave and narrow, its 

 ratio in the base of the skull varying from 4^ to 64; the 

 mesethmoid processes are always directed more or less 

 upward; and the ventral process of the basisphenoid, the 

 cranial ridges and the spines are strongly developed. 



These two groups of characters would furnish an am- 

 ple basis for the division of the genus into two, if the 

 species mentioned were alone to be considered. 

 But between the two groups distinguished by these 

 characters lies another { intranio-cr -aurora ) in which 

 the base of the skull is somewhat curved (approaching 

 straightness), the interorbital space is flat or slightly con- 

 cave, of medium width, 4 to 4^ into the base of the 

 skull, the processes of the mesethmoid are directed but 

 little upward and the ventral process of the basisphenoid 

 is poorly developed. By the interposition of this group it 

 is possible to arrange a series from -paucispinis to rosaceus 

 in which there is an almost perfect gradation of all the 

 above-mentioned characters, from strikingly curved to 

 straight base of skull, from convex and broad to concave 



