118 AQUATIC COLEOPTERA. 
specimens found by M. C. van Volxem at Rio Janeiro probably 
belong to the same species as the Santa Cruz individuals, but they 
are in such à condition of decay, that they cannot be properly 
examined. Specimens from Entre Rios, and Porto Novo do 
Cunha, are rather broader, have the upper surface more comple- 
tely covered with the brassy tinge, and the punctures on the elytra 
are a little finer; but these specimens I cannot consider to be a spe- 
cies distinct from the Santa Cruz one. I have two or three other 
closely allied species from South America in my own collection. 
The species is easily distinguished from G. ovatus Aubè by the 
completely rounded outer hind angle of the elytra. 
9. Gyrinus ovatus Aubé. Three specimens from Santa Cruz 
seem to be small individuals of this species. 
6. Gyrinus derasus n. sp. Ovatus, sat conveæus, latus, haud 
elongatus, supra versus lalera olivaceo-æneus, versus suturam 
nigro-chalybescens, sutura ænea; subtus niger, elytrorum mar- 
gine nigro-æneo, pedibus piceis ; elytris margine explanato lato, 
angulo externo obluso, minus rotundato, apice haud marginato; 
versus lalera seriebus punclorum obsoletis. Long. 6 3/4 mm. 
lat. 3 3/4 mm. 
This species is closely allied to Gyrinus chalybeus (Perty) Aubé, 
but will be easily distinguished by the above diagnosis. 
M. C. van Volxem found four individuals of this species at Bar- 
bacena. 
HYDROPHILIDÆ. 
1. Hydrophilus ater Fab. Santa Cruz 10 to 17. 10. 72. 
2. Tropisternus collaris (Fab.) Cast. Rio Janeiro, Santa 
Cruz : a considerable series shews much variation in the extent 
of the metallic mark of the thorax, and less as to the lines of the 
elytra. M. van Volxem found a very remarkable form at Buenos 
Ayres, in which these marks are very much reduced in extent, 
as this form also shews some other differences, it is highly pro- 
bable that it is a different species, but the difficulties in determi- 
ning the species of Tropisternus are so very great, that it would 
be difficult to decide such a point. 
3. Tropisternus lateralis Fab. (Æydrophilus.) Buenos 
Ayres, Santa Cruz, Rio Janeiro. In the case of this species, and 
also in others of the genus, I find extreme difficulty in guessing 
whether the different forms existing in collections form numerous 
species or only a few highly variable ones. M. van Volxem's spe- 
cimens represent two or three varieties. 
