On Aquatic Carnivorous Coleoptera or Dytiscide. 605 
917. Rhantus marginatus, Steinh. Atti. Soc. Ital. Sci. Nat. XII, p. 250.—Ovallis, 
colore variabilis, testaceus, pectore plus minusve nigricante, abdomine, vertice, 
prothorace anterius et posterius elytrisque nigris, his margine externo lineisque 
seepius obsoletis testaceis, punctis seriatis parum conspicuis, ad apicem truncatis ; 
prosterno in medio valde compresso-elevato, anterius rotundato declivo. Long. 9, 
lat. 4 m.m. 
The male has the basal joints of the front tarsi incrassate and compressed, and 
furnished beneath with long hairs, and with minute palettes; their claws are 
short, and the front one is slender and nearly straight, while the hinder, though 
about of similar length is thicker, and is strongly curved and armed with a large 
tooth beneath near the base ; the middle tarsi have the basal joints incrassate and 
greatly compressed and clothed beneath in a similar manner to the front feet ; 
the apical ventral segment is almost without sinuation at the sides, and is longitu- 
dinally strigose. The female seems to be considerably larger and broader than the 
male, and has the hind coxze more yellow, and the elytra very finely and obsoletely 
punctulate, while in the male they are nearly smooth. 
South America, (Monte Video). 868. 
IJ. 11.—Group Cotympertnt. 
The semi-membranous side piece of the first segment of the hind body (interposed 
between the stigma and the edge of the ventral segment) marked by transverse 
rugee or furrows; setigerous punctures of hind femur, when present, forming an 
irregular patch at the extremity, widely separated from the posterior margin. 
The four genera may be thus distinguished :— 
Ventral side pieces broad (as this character cannot be seen without raising the 
wing-cases, it may be supplemented by these, viz. :—upper surface con- 
spicuously reticulate ; wings of metasternum rather short); two species 
peculiar to North America. 
SCUTOPTERUS, 
(Vide p. 606.) 
usually margined at sides ; size not more than 
| 15 m.m. long ; last joint of hind tarsus sub- 
equal to the preceding joint ; many species with 
great distribution. 
RHANTUS, 
Side-piece of fourth ven- (Vide p. 607.) 
tral segment narrow ; 
upper surface not reti- 
culate, but wing-cases Metasternal groove very narrow and indistinct ; 
; COLYMBETES, 
either nearly smooth, eighteen species, chiefly boreal. (Vide p. 624.) 
transversely aciculate, 
or quasiimbricate (sub- | Metasternal groove moderately distinct; size 
tuberculate). | large (20 m.m. long) ; swimming legs elongate, 
MELADEMA, 
the last joint of their tarsi longer than the (Vide p. 631.) 
preceding one. Two Mediterranean and At- 
| 
f 
J 
( Metasternal groove broad and definite ; thorax 
J 
] 
| 
lantic island species. J 
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