NORTH AMERICAN COLEOPTERA. 279 



A revision of the SPH.ERIDIINI inhabiting 

 Boreal America. 



BY GEO. H. HORN, M. D. 



The species known to inhabit our fauna have nev^er received special 

 study, the " Synopsis of the Hydrophilidse" hy Dr. LeConte, ending 

 practically at this tribe with the description of a few new species. 

 A new study of some of the genera allied to Philydrus has made it 

 advisable to continue the investigation with the hope of arriving at 

 a clear understanding of the relationship of the two tribes. 



The most interesting and instructive structures are found on the 

 under side of the body along the median line, especially in the modi- 

 fications of the various forms and developments of the three sternal 

 pieces. 



The mesosternum is especially worthy of attention as we here find 

 an explanation of the partially develo})ed structures of some of the 

 species of the tribe Hydrobiini. 



In the preceding paper I have endeavored to show that the ine- 

 sosternal elevation is of two distinct types — first, that forming a true 

 longitudinal lamina, of which Philydrus is the type, and secondly the 

 transverse ridge and its modifications, which at times assume a j^seudo- 

 lamiiuir form. In this latter series Cymbiodyta and Hydrobius fur- 

 nish typical examples, although the modification of form in the 

 former is gradual, its greatest development occurring in C- marginella 

 and continuing still more in Helocombus. Attention is especially 

 directed to these as occasion will be taken later on to refer to them. 



Among the genera of the pi'esent tribe Cercyon alone has the 

 laminate form of mesosternal elevation, all the others having a 

 modification of the second type. In order to realize the relationship 

 of the mesosternal forms other than Cercyon to the modification of 

 the transverse ridge, it is merely necessary to trace the modifications 

 already referred to in Cyrtjbiodyta and Helocombus, in which there 

 is a marked pyramidal elevation, entirely free and distant from the 

 metasternum. If we now analyze the meso-raetasternal elevation in 

 PhiTenonotum it will be observed that the mesosternal j)ortion by 

 itself is quite that of the two genera above mentioned. In PhiBUo- 

 notum the mesosternum forms merely a small button on the end of 



