234 On Aquatic Carmvorous Coleoptera or Dytiscide. 
were not locked together by any mechanical contrivance. But in those cases where. 
the coxse were More approximate, the accurate adaptation of the parts at this 
important spot was much less easily effected owing to the more complex faces to be 
co-adapted ; and the locking together of the pieces was effected by the more tardy 
but superior process of growth of the prosternal process to rest in a groove of the 
metasternum, and by the fitting together of numerous less superficial parts and 
edges; this plan probably placed the original developers of it for a period at a 
disadvantage, but permitted ultimately a development of the parts consistent with 
greater general activity and with a consequent general higher organization. In 
the cases where the coxee are absolutely contiguous (as in Vatellini, Andex and a 
few others) space was wanting to permit of the penetration of the prosternal process 
between the middle coxe and so the prothorax was fixed by a short process being 
received into the mesosternal fork, leaving a very incomplete joint; or (as in 
Sternopriscus the prosternal process grew backwards under the projecting middle 
coxee (instead of between the coxz) and so only connected with the metasternum 
in a very imperfect manner. However this may have been, I believe that the 
distance between the middle coxze has in each species been very constant during 
very long periods of its evolution, and may prove of much assistance in deciding as 
to the relationship of particular fossils to particular existing forms. 
Hinp Coxa.—Although truly a part of the leg, yet the hind coxa in the Dytiscidee 
is developed to such an extent that it frequently forms a considerable portion of 
the external skeleton, and is therefore actually a part of the body rather than of 
the limb. It is from a taxonomical point of view, the most important part of the 
Dytiscide, and its various modifications are very interesting on account of their 
great variety in detail and uniformity in more important particulars. 
The pair of hind coxze extend all across the under surface of the body, and thus 
largely separate (when an undissected individual is looked at) the hind border of 
the metasternum from the ventral plates of the hind body. The coxa is completely 
destitute of mobility, for it is soldered in front, along its entire breadth to the 
metasternum, while behind it is accurately fitted to the basal ventral segment, and 
in the Bidessini is completely soldered thereto, so that in this group we find that 
the whole of the middle of the lower surface of the body—from the middle legs to. 
the end of the third ventral segment, consists of a single, solid, hard piece; in all 
other Dytiscidee, however, the ventral segments are not soldered to the coxe; 
although the apposition between the two parts is so perfect that they look as if” 
they were but one, yet really great mobility exists at this articulation. Each coxa 
articulates with five distinct pieces of the skeleton, viz., 1, its fellow coxa, 2, the 
metasternum proper, 8, the metasternal episternum, 4, the metasternal epimeron,. 
and 5 the basal ventral segment. It consists of two distinct parts, viz., 1, the 
lamina, interior, and 2, the lamina exterior. The internal lamina it is that carries. 
the articulation of the swimming leg, and the two internal laminz are accurately 
