On Aquatic Carnivorous Coleoptera or Dytiscide. 221 
bister scutellaris (No. 1101): these facts suggest that it is just possible that the remote 
and less perfected ancestors of Cybister scutellaris may have possessed a prothorax 
somewhat similar to that of Pelobius. The prothorax of the Haliplide is likewise 
intermediate between the Carabide and Dytiscide, its general shape being 
Carabideous, while it possesses a large prosternal process and the coxal cavities are 
closed behind, by what may be called a horny membrane so as to be in these 
respects nearer to the Dytiscide ; it shows however characters extremely different 
from Pelobius, the front coxe being short and globular, instead of elongate as in 
Pelobius, the anterior margins destitute of ciliz, and the prosternal process broad, 
and quite truncate behind instead of acuminate, 
In the Carabidee the peculiar genus Omophron has been supposed to be in respect 
of its prothorax an approximation to the Dytiscidze, but this is not really the case, 
for the coxal cavities are closed behind in a manner thoroughly Carabideous, 
and quite different from what we find in Dytiscide, Haliplidze, Pelobius, and 
those Carabidee which approximate them (Trachypachys and Systolosoma). 
The prothorax of Silphomorpha (Carabidze) in some respects approaches that 
of the Dytiscidee, but in others it is very different; the broad roof-like sides, and 
the existence of a prosternal process, and the absence of ciliation on the upper 
front margin are the points of approximation, while the form of the prosternal 
process, the mode of closure of the coxal cavities, and the very short coxe, are very 
different from what we find in the Dytiscide. If, however, the prothorax of a 
Carabus, of an Agabus and of a Silphomorpha were taken for comparison, I think 
we might quite as justly conclude that the thorax of the water beetles was 
intermediate between those of the two Carabidee, as that any other linear arrange- 
ment of the three was correct. 
In concluding this slight sketch of the structure of the prothorax in the Dytiscidee 
and some of the forms that approximate to the family, I may be pardoned for pointing 
out that any attempt to account for the resemblances we find to exist by a theory of 
heredity or community of descent only leads one into a maze of perplexities, which 
becomes the more distressing the farther we pursue it. If, however, we attempt to 
account for the resemblances as being adaptations, either by evolution or special 
creation, to the conditions of existence, or the environment, we find them quite 
comprehensible ; in the beetles inhabiting water, viz., Dytiscidee, Amphizoa and 
Pelobius, it has been necessary to exclude the water from the interior of the body, 
and to fix the prothorax solidly so that it shall not be moved from its place during 
locomotion ; thus we find perfect coadaptation of the parts to suit those of the 
afterbody, and the locking of the parts together by a prosternal process reaching 
to a part behind the articulation required to be rendered secure ; moreover in the 
higher forms which move much and rapidly in the water we find all projections anc 
prominences that would impede forward motion diminished or removed, and it 3s 
possibly for this reason that there is an absence of the cilize of the anterior margin, 
2G 2 
