NORTH AMERICAN COLEOPTERA. 239 



In the i>rece(ling table all those characters have been introduced 

 which seem to have been deemed of importance by Dr. Sharp or 

 Mr. Bedel, the object of the tabular arrangement being to avoid 

 unnecessary repetition and to render the genera more easily compre- 

 hensible in their mutual relations. 



To the acute observation of Dr. Sharp we owe the discovery of 

 two really important characters — the structure of the tai'si and the 

 form of the pseudo-basal joint of the maxillary palpi. 



In all the members of the tribe Hydrobiini the first joint of the 

 middle and hind tarsi is very short and not easily observed, but in, 

 two of the above genera the first joint has entirely disappeared, 

 having, doubtless, become completely fused with the second without 

 trace of suture. 



The curving of the pseudo-basal joint of the maxillary palpus 

 with the convexity forward in Philydrus and the reverse in other 

 genera, is a well marked character of great utility after those species 

 are removed from consideration which have heteromerous tarsi, as in 

 several Cymbiodyta that joint is quite straight. 



The genus Fhilydrus as limited above, seems to be a very homo- 

 geneous assemblage of species, all of them having a well marked 

 mesosternal lamina (with exception of ochracens), while several 

 have the prosternum more or less carinate. It is well to observe 

 here that Pliilydrus is the only genus with a well marked mesosternal 

 lamina, as in all the other genera in which a mesosternal prominence 

 exists the form is traceable in its derivation to a transverse ridge 

 (PL iii,fig. 18). 



Philydrus is the most numerously represented genus in our fauna 

 and contains all those referred to it in my former paper, excepting 

 those hereafter mentioned. 



Sindolus Sharp is not represented in our fauna. 



Helochares Muls., as restricted by Sharp and Bedel, is represented 

 by maculicollis. 



Chasmogenus Sharp, with the mesosternum merely protuberant 

 and the meutum semicircularly emarginate in front, is represented 

 by nurinatus, in whicli, however, the emargination of the mentum is 

 rather less than the description of Dr. Sharp indicates, and there is 

 no smooth submental area. The prominence of the mesosternum is 

 variable, individually, as has been observed by Dr. Sharp, in some 

 being merely an obtuse protuberance, in others quite acute, so that, 

 as he observes, there is really a rudiment of a lamina. Notwith- 



