926 On Aquatic Carnivorous Coleoptera or Dytiscidce. 



development of the coxae is sometimes extreme ; thus in Desmopachria the hind 

 coxa almost reaches the middle articular cavity. 



In other respects very important differences exist between the various components 

 of the aggregate. 



Tyndallhydrus possesses a j^rosternal process that does not connect with the 

 metasternum, agreeing in this respect with the VatelUni, (of the Dytisci fragmentati,) 

 and with Andex (of the Hyphydrini). 



In most of the species of Bidessus, the mesothoracic epimeron is very slender and 

 even at its upper posterior (episternal) angle is so little enlarged that it may be 

 almost described as linear ; on the other hand, in Pachydrus, it is comparatively 

 well develoj^ed at the point just mentioned. 



In Desmopachria the parts of the jjrosternum are reduced to a minimum of size, 

 forming in fact merely a slender frame for the encasement of the front coxaj ; the 

 anterior piece of the presternum is almost as short as possible, the band between 

 the coxae is extremely slender and the prosternal process is minute. In Huxelhydrus 

 the presternum is less reduced than in any of the other genera of the aggregate. 



In Bidessus, Huxelhydrus, (and probably in Tyndallhydrus), the mesosternal fork 

 does not reach the intercoxal process of the metasternum, whereas in the other genera 

 this connection between these two parts is well completed. 



Bidessus, Huxelhydrus, and Tyndallhydrus have the slender hind tibiae a little 

 bent near the base, and becoming a littJe thicker from the base to the apex, 

 whereas in the other genera the tibiae are stouter, and are straight, and .of about 

 one thickness from base to apex. 



The three genera, Heterhydrus, Pachydrus, and Desmopachria, by several char- 

 acters appear to form a naturally distinct aggregate from the other three, thus they 

 are of peculiar short, broad, form, with a broad prosternal process, with thicker 

 swimming legs, and straight tibiae ; short and compressed or subserrate antennae, 

 and completely separated middle coxEe ; and with contiguity between mesosternal 

 fork and metasternal intercoxal process : whereas in the other genera, the form is 

 more oblong, the prosternal process oblong, the middle coxaj contiguous, and the 

 mesosternal fork disconnected from the metasternal process, the liind tibiaj differently 

 formed, and the antennas comparatively filiform. Although these characters are 

 very considerable, yet the multitude of species included under Bidessus show much 

 variety of form, so that it is possible that when carefully examined they may 

 present intermediate forms. 



The Bidessini are distributed over all lands, but apparently prefer the warmer 

 regions. Tropical America seems to be the metropolis of the Pachydrus group of 

 genera, Australia of the Bidessus group. No species has yet been found in the 

 Pacific Islands. 



I should remark that I have not been able to ascertain with certainty that the 

 hind-body is actually soldered to the coxa in Heterhydrus and Tyndallhydrus, but I 

 entertain almost no doubt that such is the case. 



