92 Mr. D. Sharp's revision of the species 



sternus quite similar in the males and females. I have 

 found, however, that in the males of a great number of 

 the species there is a small tooth or spine on the under 

 surface of the inner claw on the hinder and middle feet ; 

 and although there are a few species in which I have not 

 detected this external sexual mark, I believe it will ulti- 

 mately be found to exist in all species of the genus. In 

 a few species there is a peculiar sexual difference in the 

 sculpture of the mentum, the males having this part more 

 densely punctate than the other sex. These are the 

 only certain external sexual characters I have been able 

 to detect. 



In the hope that the structure of the cedeagus might 

 afford a means of distinguishing the species, I have 

 examined it in several forms selected for the purpose, 

 but I find that not only is this organ remarkably minute 

 in the species of this genus, but that it is also exces- 

 sively similar even in very widely different species. 



The difficulty of distinguishing the species is increased 

 by the fact that two forms of what I may call adventitious 

 sculpture exist in certain individuals of several species. 

 The first of these peculiar sculptures is a pitting of the 

 surface, giving rise to a variolose appearance, and often 

 occurring with great regularity over the whole of the 

 upper surface ; this peculiarity, which is not, I believe, 

 variation, but more probably dependent on some physical 

 condition to which the specimen possessing it is sub- 

 jected during its metamorphosis from the pupal condition, 

 occurs in numerous other Hydrophilidce and Dytiscidce, 

 and is not unfrequently alluded to by describers as being 

 a specific character. The second form of adventitious 

 sculpture consists in the appearance in certain individuals 

 of two lines of punctures near the outer margin of the 

 wing-case, or near the suture ; this character also can 

 scarcely be considered variation, but I believe it depends 

 on the fact that although the species of the genus 

 appear externally to be without series of punctures on 

 the wing-cases, yet in reality these exist in the interior 

 of this part (they are in fact more or less conspicuous 

 on the inner face of the wing-case according to the con- 

 dition of the membrane covering this face) and become 

 occasionally evident externally if the chitinous sub- 

 stance be a little thinner than usual, or from some other 

 cause. 



After eliminating these sources of confusion, and after 



