On Aquatic Carmvorous Coleoptera or Dytiscide. 889 
_A comparison of Agabinus and Platambus 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 
coxee, and the superior swimming legs, and it is more strongly differentiated from 
Agabus by the broad epipleuree 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, differentiated from average Agabides in similar manners, 
by the widened epipleurze and straightened coxal lines, we must say that, as regards 
the epipleuree, Agabinus is intermediate between Agabides and Platambus, while 
as regards the coxal lines and processes the reverse is the case, Platambus being 
intermediate between Agabides and Agabinus. 
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, approximating 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 searcely 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 Dytiscidee the development of 
the coxal notch was a mode of differentiation prior to differentiation in the ceneral 
form of the coxal processes. 
This insect 1s peculiar to California. 
I. 52.—Genus PLATAMBUS. (Vide p. 548.) 
This aggregate comprises three species distinguished by the structure of the 
epipleurze 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 before 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 coxee large, with the front border considerably arched, and approaching 
nearer to the middle coxa than it does in Agabus (even in group 14 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 setee 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 angies of the prothorax a little 
