1918.] F. H. Gravely : Passalidae of the World. 9 



Mastochilus capitalis, 1 Blackburn, 1900, pp. 209-210. 



„ subobliquus, Tryon, 1892. 



Pelops triumphator, Zang. 19016. pp. 182-181. 

 Plesthenus mandibularis, Heller, 1900, p. 11, pi. fig. 5. 



scutelhpunctatus, Zang, 1903a, p. 339. 

 Protomocoelus sternbergi, Zang, 1905a, pp. 236-238. 

 Tristorihus papuanus, Heller, 1910, p. 15, pi. figs. 15-15a. 

 Tristortlius punclicollis, Heller, 1916, p. 353. 



In view of the striking correlation which has been shown to exist, in certain Indo- 

 Australian Passalidae, between classification and distribution (Gravely, 19 146). it is natural, 

 when seeking for sound principles on which to base the classification of the family as a 

 whole, to consider first whether it may not be possible on purely morphological grounds to 

 regard the American and Ethiopian Passalidae respectively as series independent both of 

 one another and of Indo-Australian forms. 



It will be remembered that among the Indo-Australian Passalidae the Aulacocyclinae 

 standby themselves, widely separated from all other forms (Gravely, 1914c, pp. 191 -192). 

 They are also widely separated from American and Ethiopian forms and need not be further 

 considered in this connection. In all other Indo-Australian Passalidae, except certain highly 

 asymmetrical ones with reduced dentition, the lowest terminal tooth is directed inwards 

 and is situated beside the anterior lower tooth, which is broadly triangular as seen from 

 above and tends to be flattened above and below. In American and Ethiopian forms, on 

 the other hand, none of which are asymmetrical, either the terminal teeth are situated in a 

 line one above another ; or the anterior lower tooth (on the right side at least) is more 

 columnar and tends to be flattened in front and behind, or may be concave behind much 

 as it the Aulacocyclinae, or both characters may be present. These characters are more 

 marked in some species than in others, but in greater or less degree they are of universal 

 application ; and they are associated with various other, often more striking, differences 

 of more limited application. 



The reduction of the dentition takes place mainly in highly specialized genera of the Indo- 

 Australian and American series, genera whose relationships are, as a rule, clearly indicated by 

 other characters. Among the former it is confined to the more highly asymmetrical genera, 

 and among the latter it is found chiefly in genera composed wholely or in part of flightless 

 species. It is easy to follow the manner in which the reduction is carried out in the former ; 

 but in the latter it is often more difficult, the reduction being more abrupt. In the Indo- 

 Australian series it is always due to the fusion of the anterior lower tooth with the lowest 

 terminal tooth (see Gravely, 1914c, text-fig. 7, p. 314). In the American and Ethiopian 

 series, with the possible exception of the genus Proculus, it seems always to be due to 

 the fusion of the two lowest terminal teeth, though this is apt to be accompanied by a 

 dorsoventral flattening of the anterior lower tooth, which thus loses its distinctive shape. 



The reduction of the dentition in the American and Ethiopian series may most 

 conveniently be illustrated by reference to the genera of the Proculejoides group of Passalinae 



1 Mastcchilus capitalis, Blackburn = Epispkenoides quaestiov**, Kmvert. See Zang, 1905c. p. 223. 



