i82 Memoirs of the Indian Mitseum. [Vol,. Ill, 



differ from one another in size or in the extent to which their antennae happen to be 

 unfurled. It is so difficult, moreover, to say exactly where the shaft of the antenna 

 ends and the lamella begins, that it often depends entirely on the point of view of 

 the observer whether a specimen appears to agree with a given description. 



Mandibles. 



The mandibles of a Passalid are triangular in section, and bear teeth on the 

 upper and inner margins as well as at the extremity. There is a striking uniformity 

 of plan in the mandibles of all species, and such deviations as occur appear as a rule to 

 be constant for whole genera rather than for individual species. These deviations 

 take ' the form of the suppression, bifurcation or duplication of particular teeth, 

 suppression being as a rule most marked in the right mandible and duplication in the 

 left, no matter which side of the head is most developed. These teeth (see text- 

 figure C I ) may be described as follows : — 



One upper tooth, situated on the upper margin of the mandible. 



Three terminal teeth. Of these the upper two are always compressed laterally; 

 the lowest one is compressed either laterally or dorso-ventrally according to the sec- 

 tion of the family to which the insect in question belongs. '^ It is set further back in 

 the latter case than in the former. 



Three lower teeth. . Of these the anterior one varies considerably in form in 

 different genera; the middle one is always a chisel-shaped lamella which is hinged 

 on to the mandible except in certain genera of Aulacocyclinae; and the posterior one 

 is a strong hollowed cusp so completely hidden betwçen the labrum and maxillae 

 that it cannot be satisfactorily examined unless the mandible is extracted. 



Upper sufface of head. 



The principal features of the upper surface of the head are shown in text-figure 

 I A. This surface is usually marked, at least in part, with a number of hair-bearing 

 punctures, which in certain species run together to form grooves. The nature of these 

 markings seems to be of greater importance than their extent. 



The head is bounded laterally above by a pair of supra-orbital ridges, which 

 extend from the anterior margin above the eyes directly backwards. These ridges 

 are usually of almost uniform height throughout the greater part of their length and 

 obhquely truncate in front. In species in which the ridge is a broad one, its crest 

 commonly forks above the truncation, one branch extending along the outer and the 

 other along the inner margin of the anterior face ; but in a few of these species the 

 inner branch is found to be more or less obsolete in certain specimens. The size of 



' The Oriental Passalidae have been found to belong to two distinct sections of the family, which 

 differ markedly from one another in many ways. One of these sections consists entirely of members of 

 the sub-family Aulacocyclinae, and it is often convenient to refer to this as the first section of the 

 family, as opposed to the remaining subfamilies which together constitute the second section. These 

 subfamilies are considered in the present paper to be four in number, and I have termed them Pleur- 

 ariinae, Aceraiinae, Gnaphalocneminae and I^eptaulacinae. See below, p. 191 and onwards. 



