On Aquatic Carnivorous Coleoptera or Dytiscidce. 889 



A comparison of Agabinus and Platanibus with a view of ascertaining if either 

 of the two forms can be considered ancestral to the other, gives a strongly negative 

 result ; which may thus be summed up. Platambus must be considered a much 

 higher form than Agabinus, because of the much greater development of the hind 

 coxae, and the superior swimming legs, and it is more strongly diiferentiated from 

 Agabus by the broad epipleur^ than Agabinus is. On the other hand the peculiar 

 coxal processes of Agabinus depart extremely from Agabus and all other Agabides, 

 except Platambus, and the latter in this respect is intermediate between ordinary 

 Agabides and the genus Agabinus. Thus looking at Platambus and Agabinus as 

 two approximated forms, difi'erentiated from average Agabides in similar manners, 

 by the widened epipleurte and straightened coxal lines, we must say that, as regards 

 the epipleurje, Agabinus is intermediate between Agabides and Platambus, while 

 as regards the coxal lines and processes the reverse is the case, Platambus being 

 intermediate between Agfabides and Aafabinus. 



Another suggestive comparison may be made between Agabinus, and the 

 Australian Hyderodes. This latter insect may be looked upon as a primitive form of 

 the group (Dytiscini) to which it belongs ; and in it we find a shape of the coxal pro- 

 cesses, apjaroximating considerably to Agabinus, but notwithstanding this general 

 resemblance of the coxal processes of the two, there is seen the important difference 

 that a deep coxal notch or cleft exists in Hyderodes, while there is scarcely any 

 trace of such to be seen in Agabinus. It appears to me that the inference we are 

 entitled to draw from this is that among primitive Dytiscidse the development of 

 the coxal notch was a mode of differentiation prior to differentiation in the general 

 form of the coxal processes. 



This insect is peculiar to California. 



T. 52.— Genus PLATAMBUS. {Vide p. 548.) 



This aggregate comprises three species distinguished by the structure of the 

 epipleurte of the elytra, which present behind the middle a flat surface that is 

 considerably broader than in other Agabini. Coxal lines very deep in front, and 

 distinctly directed outwards befure their termination, so that the coxal lobes are 

 quite distinct, but have a less extension in the transverse, and a greater in the 

 longitudinal direction than they have in other Agabini. 



Hind coxse large, with the front border considerably arched, and approaching 

 nearer to the middle coxa than it does in Agabus (even in group 1 4 of that genus) ; 

 the wings of the metasternum therefore very short and sublinear. Prosternal 

 process very broad. Swimming legs moderately stout, but their femora with the 

 postero-external angle rounded, and its setae but little developed. 



The three species of which this aggregate is at present composed, all have the 

 upper surface variegate with yellow and the hind angles of the prothorax a little 



