On Aquatic Carnivorous Coleoptera or Dytiscidoe. 929 



The genus Hydroporus departs however in this respect from the other Hydro- 

 porini, and possesses the more perfect and more usual structure. As the connection 

 between these two parts is undoubtedly a point of perfection, Hydroporus must, 

 other points being assumed as of equal perfection, be considered a higher form 

 than the other genera, in which this connection is not attained. The line of 

 demarcation between the two categories, is however not an absolute one, for in 

 those genera where the connection of the two parts does not exist, there are some 

 species where it is nearly, although not completely attained ; and in point of fact, 

 most if not all of the species of the Hydroporini may be considered to be in process of 

 gaining this connection, for they show a rudimentary process, more or less developed 

 in different species, and if this increases or continues to grow, the connection 

 between the two parts of the breast will be completed. 



I fail to understand this process unless it be considered as a growth that will have 

 this result. 



Not only are the species in which the connection does not exist variable as 

 regards their greater or less degree of approximation to its attainment ; but those 

 where the connection has been attained show it in various degrees of perfection. 



The functional result attained by the union is perhaps some power of mobility of 

 the prothorax by extension ; while when not extended, the mutual adaptation of 

 contiguous parts, and their power of resisting strains is increased. Hydroporus 

 oblongus may be taken as a type of the most perfect form of the union attained by 

 these parts ; in it the apices of the sides of the mesosternal fork are brought to just 

 the same plane or level as the middle of the metasternum, and the apex of the 

 metasternal process is prolonged into the groove of the fork, and its anterior 

 portion is grooved or shows an impression for the apex of the prosternal 

 process. 



In other species, such especially as are more convex beneath, as H. cimicoides 

 (No. 501) and its allies, the junction is effected by the apex of the metasternal 

 process being recurved and resting on a prominence that projects from the back of 

 the mesosternal fork ; the adaptation of the parts being thus much less perfect 

 than it is in H. oblongus. 



In Dytiscus dorsalis (No. 630) the mesosternal fork is elongate and has even 

 grown out a little beyond the general plane of the metasternum, the intercoxal 

 process of which is therefore a little arched, and the junction is effected by the edge 

 or extremity of the two pieces of the mesosternal fork having grown backwards on 

 each side of the apex of the intercoxal process of the metasternum. 



We are entitled I think from these facts to conclude that, just as the junction in 

 the genera where it does not yet exist is possibly being effected by the species inde- 

 pendently of one another, so where it does exist it has likewise been attained in 

 diverse manners by slightly different modes of growth, and does not indicate any 

 genetic connection between the species possessing it. 



