1918.] F. H. Gravely: Passalidae of the World. 125 



through the fusion of the two lowest terminal teeth. In Indo- Australian subfamilies it is 

 always associated with cephalic asymmetry and never with the loss of the habit of flight, 

 and comes about through the fusion of the anterior lower and lowest terminal teeth 

 (pp. 9-10, fig. ii). 



The loss of the habit of flight appears to allow of greater specialization of the wings as 

 stridulating organs. It produces definite structural modifications in the insect (pp. 4-5). 

 The following genera contain, so far as is known, only flightless forms : — Cylindrocaulus, 

 Platyverres, Pleurarius, Proculejoides, Proculejus, Procululus, Proculus, Pseudacanthus 

 and Publius. The following species are also flightless : — Labienus moluccanus and gigas, 

 Macrolinus obesus and ursus, Passalus quitensis and V index synelytris. 



The structure of the male genital tube is almost uniform throughout the family. 

 The genera of Aulacocyclinae other than Ceracwpes and Cylindrocaulus differ, however, from 

 the rest of the family in that the basal piece and lateral lobes are represented by one 

 undivided plate (p. 5). 



The central tubercle is usually larger in females than in males in species in which 

 it varies greatly in size (p. 5). 



2. Classification. 



Seven subfamilies have been recognized, of which one, the Aulacocyclinae, confined to 

 the Indo-Australian area with China and Japan, is somewhat widely removed from all 

 t)he others (p. 9). Two others are confined to the Indo-Australian area. These are 

 distinguished from American and Ethiopian subfamilies by the structure of the mandibles 

 (p. 9). The Ethiopian subfamily is distinguished from the four American ones by the 

 structure of the anterior margin of the head (pp. 10-1 1). 



The number of genera has been greatly reduced. Specific synonymy has not been dealt 

 with, but there can be little doubt that a similar reduction is required in the number of 

 species. 



3. Geographical Distribution. 



Passalidae appear to flourish only under more or less moist tropical conditions. 



American, Ethiopian and Indo-Australian forms belong respectively to different 

 subfamilies, probably without exception (pp. 9-12). 



The group of Macrolininae with strong asymmetrical tendencies is of special zoogeo- 

 graphical interest. Its most primitive species inhabit Ceylon and Australia. These 

 .are closely allied to one another but give rise to divergent lines of descent, confined 

 respectively to the Oriental Region with Celebes, and to the Australian Region. Both these 

 regions are composed of a series of smaller areas, each characterized by a distinct Passalid 

 fauna, which is more highly specialized in those nearer to Celebes than in those further 

 away. These areas are : in the Oriental Region — Ceylon, the Indian Peninsula, the 

 Indo-Chinese Subregion and the Malayan Subregion ; and in the Australian Region — 

 Australia, New Guinea and the Moluccas (pp. 120-124). These facts bear out the 

 suggestion (Gravely, 19 13, p. 204) that conditions towards the centre of the Archipelago are 

 peculiarly favourable for evolution, and that as more and more highly specialized forms 

 iave arisen there, they have migrated outwards, driving before them the less highly 



