On Aquatic Carnivorous Coleoptera or Dytiscida. 227 
pieces to the mesothorax than have the other Carabide. In none of them, 
however, do I find the abbreviation to be so considerable as it is in the 
Dytiscide ; 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 Dytiscidea —Amphizoa. In describing the medisternum (ante page 223) 
IT remarked that its central column appeared to consist of two distinct parts, and 
further that the inner or front margin of the episternum had also the appearance 
of being a distinct piece connecting with the front piece of the central column. 
In the Carabide 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 
Carabid differs from that of the Dytiscidee. As the structure of the mesosternum 
in the Carabidee has not so far as | 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 Dytiscide 
differs from that of the Carabidz, 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 tribe Noterides, as well as in some Hydroporides, of the Dytiscide, 
the epimeron is as small and linear as it is in most of the Carabide ; and 
in Silphomorpha of the Carabide, the shape and size of the epimeron is similar to 
the Dytiscidee, (except that it does not penetrate to the coxal cavity). Fourth, 
the rule in the Carabide is that the intermediate coxe are very distinctly separated, 
but in the Dytiscide they are more approximate ; this however is liable to numerous 
exceptions ; in the Pseudomorphides (and according to Lacordaire in the Ozcenides), 
the middle cox are very approximate as in the Dytiscide, and the Dytiscidz 
themselves differ considerably in this respect, thus although the cox are con- 
tiguous in Vatellini, they are in Hydrocanthini and Suphisini as widely separated 
as they are in Carabide ; and even in some of the higher forms—as the Hydaticini 
—the separation of the middle coxze is moderately broad. Fifth, in the vast mass 
of the Carabidx, the mesothoracic epimeron does not penetrate to the coxal cavity, 
whereas in the Dytiscide it invariably reaches the cavity; but the first, or 
fragmentary, series of the Carabide resemble the Dytiscidze 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 
Dytiscidee ; second, the mesosternum of Omophron is very similar to that of the 
Noterides ; third, although the Pseudomorphides resemble the Dytiscide as regards 
the mesosternum, inasmuch as their coxee are approximate, and that sometimes 
TRANS. ROY. DUB. SOC., N.S. VOL, II. 2H 
