118 On a Parasitic Copepod of the Clam. [February, 
in great numbers, while other years they are very scarce. During 
the years 1878 and 1879 Arvicola riparius was very common and 
could be found in every locality, but in 1880 most of them disap- 
peared, and for a long time they were very scarce. They have 
slowly increased in numbers and are now as numerous, perhaps, 
as ever. Whether these strange reoccurrences are the result ot 
migrations or disease we are, from the present state of our know- 
ledge, unable to determine. 
Mice have their enemies, as do most other animals. They are 
caught in large numbers by the smaller hawks (Zinnunculus spar- 
verius Vieill, Accipiter cooperi Bp., and Accipiter fuscus Bp.), 
owls (Scops asio Bp., and Asio accipitrinus Newton); cats and 
dogs catch them as opportunity affords. Many also are killed 
by their curious little enemies, the shrews. 
In habits no two of the species named approach each other 
very nearly except in general characteristics. They all appear to 
be gregarious, living as a rule in colonies. The pine mouse 
burrows deepest, and makes the most lengthy runways. The 
passages made by Cooper’s field mouse are never of much length, 
but are very sinuous and intricate. In food each species appears 
to partake of some particular kind or kinds found near the local- 
ity it-frequents. Except in case of the pine mouse and Cooper’s 
mouse the localities occupied by each species do not appear 
to overlap, each frequents a peculiar kind of region wherein it plays 
its part in the economy of nature. 
ON A PARASITIC COPEPOD OF THE CLAM. 
BY PROFESSOR R. RAMSAY WRIGHT. 
INCE the researches of Dana, published between thirty and 
forty years ago, comparatively little attention has been given 
to the Copepoda in America. So much is this the case that 
Gerstzcker in his account of the geographical distribution of the 
order, mentions only sixteen species as inhabiting the fresh 
waters and coast region of North America, the described forms 
being all fish parasites. Of late, however, important contribu- 
tions to the knowledge of the fresh-water, free-living forms have 
appeared in this journal, and new parasitic species have been 
1 Bronn’s Thierreich, Vol. v, c. 1876, p. 799. 
2S. A. Forbes. Entomostraca of Lake Michigan, Vol. xvi. 
C. L. Herrick. Heterogenetic development in Diaptomus, Vol. xvir. 
