On Aquatic Carnivorous Coleoptera or Dijtiscidoe. 227 



pieces to the mesotliorax than have the other Carabidse. In none of them, 

 however, do I find the abbreviation to be so considerable as it is in the 

 Dytiscidte; in Systolosoma for instance (the only one of the three genera just 

 mentioned that I have been able to dissect) the middle of the mesosternum 

 is decidedly more elongate than it is even in the most Carabid like of 

 the Dytiscidaa — Amphizoa. In describing the medisternum {ante page 223) 

 I remarked that its central column appeared to consist of two distinct parts, and 

 fui'ther that the inner or front margin of the episternum had also the appearance 

 of being a distinct piece connecting with the front jjiece of the central column. 

 In the Carabida3 these pieces are still more distinct, and it is in fact by the much 

 larger size of these front pieces of the medisternum that the mesosternum of the 

 Carabidaj differs from that of the Dytiscidse. As the structure of the mesosternum 

 in the Carabidee has not so far as I am aware been thoroughly examined, I must 

 for the present leave this subject, merely remarking that in Carabus catenulatus 

 the middle of the mesosternum appears certainly composed of three parts super- 

 posed one on the other. The third point in which the mesothorax of the Dytiscidas 

 differs from that of the Carabidaj, is the greater development of the posterior side 

 piece in the former group ; this is, however, by no means constant, for in the whole 

 of the ti'ibe Noterides, as well as in some Hydroporides, of the Dytiscidie, 

 the epimeron is as small and linear as it is in most of the Carabidse ; and 

 in Silphomorpha of the Carabidse, the shape and size of the epimeron is similar to 

 the Dytiscidse, (except that it does not penetrate to the coxal cavity). Fourth^ 

 the rule in the Carabidas is that the intermediate coxse are very distinctly separated, 

 but in the Dytiscidse they are more approximate ; this however is liable to numerous 

 exceptions ; in the Pseudomorphides (and according to Lacordaire in the Ozoenides), 

 the middle coxse are very approximate as in the Dytiscidse, and the Dytiscidse 

 themselves differ considerably in this respect, thus although the coxse are con- 

 tiguous in Vatellini, they are in Hydrocanthini and Suphisini as widely separated 

 as they are in Carabidse ; and even in some of the higher forms — as the Hydaticini 

 — the separation of the middle coxse is moderately broad. Fifth, in the vast mass 

 of the Carabidie, the mesothoracic epimeron does not penetrate to the coxal cavity, 

 whereas in the Dytiscidaj it invariably reaches the cavity ; but the first, or 

 fragmentary, series of the Carabidse resemble the Dytiscidse in this respect, the 

 epimeron reaching to the cavity as in the water-beetles. 



Turning to the special points of approximation between the two families in 

 respect of the mesosternum we find, first, that there is but little difference between 

 the mesosternum of Systolosoma and Amphizoa, still the former is a little more 

 Carabidiform than the latter in this respect ; and both differ but little from the 

 Dytiscidse ; second, the mesosternum of Omophron is very similar to that of the 

 Noterides ; third, although the Pseudomorphides resemble the Dytiscidse as regards 

 the mesosternum, inasmuch as their coxse are approximate, and that sometimes 



TBANS. ROT. DUB. SOC, N.S. VOL. II. 2 H 



