AMERICAN COLEOPTERA. 235 



common to all the species of the genus, there are others found in 

 various portions of the body peculiar to and at times definitive of 

 groups. These occur in first, the mandibles ; second, the labrura ; 

 third, the thorax; fourth, the elytra; and may be again tabulated in 

 the following manner. 



Mandibles. — Normal form : Similar in the sexes and both sides 

 equal. Abnormal form : Right mandible of male furnished with a 

 tooth, of greater or less length, on the lower margin near the tip, 

 {dorsn/is, marginata and hamata). 



Labrum. — This organ may vary in length in the sexes, that of the 

 female being longer, {unipunctata, longilahris), or in color, white in 

 the male and black in the female, (Jongilahris, var. montana, Pilatei, 

 striga). In some species (scuteUaris and its races), the labrum varies 

 in color regardless of sex. The teeth of the labrum are sometimes 

 more marked in the female, as in striga. 



Thorax. — The normal form may be either trapezoidal, narrower at 

 base, or very nearly cylindrical, and in each case similar in the sexes. 

 In many species the hind angles become prominent, and the base 

 broader than the apex or as broad, and the transverse basal impression 

 deeper, (hirticoUis, dorsalis, hamata and marginata). These are all 

 maritime species. 



Elytra. — In nearly all the species the usually more robust form 

 of the female enables that sex to be distinguished at a glance, the 

 form is, however, identical in the sexes. The differences worthy of 

 comment are, those (1), of general form, (2), the sub-apical sinuation, 

 or (3), the tip and sutural angle. While the males have the normal 

 form, the females have the elytra much more broadly arcuate at the 

 sides in all the maritime species above noted, and rather strongly sub- 

 angulate behind the humeral angle in dorsalis. These species furnish 

 the only examples of marked difference in outline. The elytra narrow 

 at tip, by being gradually arcuate or oblique, and while these typical 

 styles merge insensibly, the latter form is gradually modified by be- 

 coming more and more sinuate, especially in the females, until a well 

 marked tooth appears varying in the degree of its acuteness. This 

 is especially marked in certain of our fluviatile species in which the 

 elytra have the additional basal spot, {cuprascens^ macra, puritana, 

 etc.) The tip adjacent to the suture may be in the form of an acute 

 angle, rectangular, truncate or rounded. In most of the species the 

 form does not vary sexually, but may be any one of the above forms 



