62 HYDROPHILIDA., 
in numerous characters, and, owing to their size and hemispherical form, having a 
superficial resemblance to Cyclonotum. The antenne are very short, the basal joint 
reaching but little beyond the side of the head ; the second joint is very closely connected 
with it, appears, indeed, to be only incompletely divided from it, and is longer than its 
exposed portion; the third and fourth joints are very minute, while the fifth forms an 
obscure auricular appendage to the base of the very laxly articulated club. The 
maxillary palpi are stout and very short, the apical joint about equal in length to the 
second joint, while the third joint is considerably shorter; the mandible is so abruptly 
curved that its apical portion forms a long slender process placed at right angles to the 
broad basal portion, and is bifid at the extremity; the mentum is large, flat, smooth and 
shining, and nearly truncate at the extremity; the labial palpi are small. The pro- 
sternum and front coxal cavities are formed much as in Hydrobius ; the former is rather 
obscurely carinate along the middle. The mesosternum bears in front of the coxe a 
large prominence elevated to the level of the metasternum, and having its lower face 
scabrous. The metasternum sends off in the middle in front an elongate slender process 
separating the middle coxe and articulating with the mesosternal process; the breast is 
finely pubescent ; but the femora are glabrous beneath, except that the middle ones bear 
some coarse punctures and rough sete. ‘The tibiz are armed with long spines. The 
ventral segments are shining and bear rather numerous fine, erect hairs. 
The characters are drawn from 0. brevicornis. The genus is in certain respects one 
of the most interesting of the Hydrophilide; the sculpture of the upper surface in 
O. brevicornis and O. vestitus is remarkable; and the maculation of the surface in 
O. maculatus with brilliant metallic spots is unique. I am not acquainted with any 
species other than those here described; and they all three appear to be rare, two of 
them, indeed, being unique. 
The nearest ally of the genus is probably the genus or subgenus Anacena, C.J. Thoms. ; 
but Oocyclus differs by the absence of the fine pubescence on the femora and ventral 
segments, and by the structure of the pieces at the middle of the breast, which 
approximate much to the structure of these parts exhibited by Cyclonotwin. 
1. Oocyclus vestitus. 
Subrotundus, supra viridescens, crebre irregulariter punctatus, in thorace elytrisque setulis sat elongatis haud 
dense vestitus; palpis antennisque testaceis, pedibus rufis; prosterno carinato, mesosterno lamina magna 
irregulari armato. 
Long. 2, lat. 17 lin. 
Hab. Mexico, Orizaba (Sallé). 
Head very short and broad, closely and rather coarsely punctate ; palpi yellow, stout, 
and very short, considerably shorter than the antenne. ‘Thorax forming a continuous 
outline with the afterbody, much narrowed towards the front in a curve, the surface 
evenly, moderately closely and coarsely punctate, and bearing rather numerous fine 
