included in the genus Tropistcrnus. 93 



arranging the species in groups, there is still much 

 difficulty in distinguishing the species, owing to the 

 variability in some other points. The most important 

 of these is the spine or carina frequently apparent on 

 the last ventral segment ; this in certain species, e. g., 

 H. apicipalpis, is very constant in its size and form, but 

 in others (especially in H. lateralis) it is excessively 

 variable, unless it be the case that more than one 

 species is mixed together in these cases of apparent 

 variability. There seems some probability that this 

 latter may be the case, because in the species last named 

 the crest is quite absent in South American individuals, 

 but is nearly always present in individuals from the 

 United States of North America. In Central American 

 individuals it is present in various degrees of develop- 

 ment. 



A fact of some interest is the presence in numerous 

 species of the genus of an excessively fine squamosity on 

 the polished surfaces of the hind tibiae and tarsi. This 

 is somewhat similar to what we find existing more con- 

 spicuously in Eretes, of the family Dytiscidce ; it is ex- 

 cessively delicate and very easily removed, and is of 

 interest to the evolutionist inasmuch as it may be con- 

 sidered to be the remnant of a structure formerly more 

 developed, but now in process of disappearing. This 

 peculiarity may be well observed in the common species, 

 T. nitens, Cast. 



It is still more interesting to observe that in the case 

 of Eretes and Tropistemus this peculiarity of the 

 squamose swimming legs is accompanied by another 

 common to the two, and almost (if not altogether) con- 

 fined to them amongst their allies, viz., the existence on 

 the epipleural margin of short rigid spines. In the 

 Tropistemi each of these spines is inserted in a large 

 puncture or depression, the shape of which apparently 

 adapts it to receive the spine when depressed, and there 

 is no doubt these spines are mobile and capable of being 

 depressed or erected; these peculiarities present some 

 valuable specific characters, and may be seen to advan- 

 tage in Hydrophilus apicipalpis. 



In discussing the characters of some of the species I 

 have made use, in speaking of some punctures on the 

 head and thorax, of the term " systematic punctures " ; 

 I allude by this to some punctures which are present 

 with great constancy in the Hydrophilidce on these parts 



