66 COLEOPTERA OF NORTH AMERICA. 



Hydrotriipes, Ilybiosoina, and Agabiims, are exclusively Cali- 

 fornian, each represented by a single species. Matus and Aga- 

 betes have been found only in the Atlantic region, the former also 

 extending to Australia. The other genera are represented on 

 both sides of the continent. Agabus includes Gaurodytes and 

 Anisomera of our lists, which have been separated on insufficient 

 characters. The species of Agabus are numerous, especially in 

 the northern regions, and, although separated by good structural 

 characters, frequently bear a deceptive resemblance to each other. 



Group II. — Colymbetes. 



The species are usually of larger size than those of the preced- 

 ing group, and may be divided according to sculpture, although 

 additional cliaracters are obvious, which can Ije referred to in 

 Dr. Sharp's memoir. 



Elytra loticulate. Scutopterus. 



Elytra smooth, or (9) with coar.se short lines, nietasternina with deep 



groove. Rhantus. 



Elytra transversely strigose, witli anastomosing lines (but not in our 



species) sometimes smooth, metasternum with fet^ble groove. 



Colymbetes. 



The species of Rhantus and Colymbetes occur on both sides 

 of the continent; Scutopterus, thus far, in the Lake Superior and 

 Hudson Bay regions; and in fact the larger number of species 

 arc northern, though a few stray into southern California. 



Tribe III.— DYTISCIIVI. 



The species of this tribe are large, or at least moderate in size, 

 never small, and are easily distinguished by the peculiar dilata- 

 tion of the front tarsi of the S ; of which, namely, the first three 

 joints form a circular pallette, with cupules on the under surface, 

 which vary in size and arrangement according to genus and 

 species. The middle tarsi are frequently dilated, the joints being 

 oblong, with variously arranged cn])nles, or suckers beneath. 

 The last two pairs of abdominal stigmata are usuall}^ large, and 

 the ruga) of the membrane around them arc well developed. 



Our genera may be tabulated as follows : — 



i 



