234 On Aquatic Carnivorotts ColeojJtera or Dytiscidce. 



were not locked together by auy mechanical contrivance. But ia 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 

 metasteruum, 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 coxse 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 coxa3 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 

 coxjB (instead of between the coxa;) and so only connected with the metasternum 

 in a very imperfect manner. However this may have been, 1 believe that the 

 distance between the middle coxse 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. 



Hind Cox^. — Although tinily a part of the leg, yet the hind coxa in the Dytiscidse 

 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 taxouomical point of view, the most important part of the 

 Dytiscidte, and its various modifications are very interesting on account of their 

 great variety in detail and uniformit}- in more important particulars. 



The pair of hind coxse 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 Dytiscidse, however, the ventral segments are not soldered to the coxsb ; 

 althouoh 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, 3, 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 laminae are accurately 



