ORTMANN: THE CRAWFISHES OF THE STATE OF PENNSYLVANIA 467 
congenial conditions in the lake. The specimens taken may have been accidentally 
swept into the lake. Thus I do not believe that it will be discovered in the part of 
the lake bordering upon Pennsylvania. 
C. immunis, however, besides being found in northern Ohio, reappears in New 
York. Faxon (1898, p. 654) has recorded it from a tributary of Oneida Lake, and 
recently I have seen specimens, belonging to the New York State Museum, col- 
lected by Mr. F. C. Paulmier in Rensselaer Lake, Rensselaer County. ‘Thus its 
presence in New York, upon which I cast some doubt (1905), p. 134), is to be 
regarded as firmly established. However, the connection of these eastern localities 
with the western range has not been discovered. If a connection is present at all, it 
is to be looked for in the Erie-St. Lawrence basin, and thus would possibly include 
the lake shores of Pennsylvania. Yet this connection may not exist, and C. immunis 
in New York may be a recent, artificial introduction, which is not altogether impos- 
sible, since we know that the crawfishes used for food in the New York market come 
in part from the lake regions (Milwaukee, see Ortmann, 1900, p. 1260), and thus 
this species may have been introduced. But this question is by no means settled, 
and we should try to obtain further facts. 
Finally we may observe that the conditions now existing in the case of the 
Pennsylvanian crawfishes may not be original, but may have been altered by human 
agency. The possible influence of canals upon the dispersion of two species, C. 
limosus in the east, and OC. obscurus in Crawford and Erie Counties, has been dis- 
cussed in the foregoing pages, and the transplantation of C. obscwrus into Wills 
Creek has been stated to have apparently occurred, accidently or intentionally, 
through human agency. No other cases of dispersion beyond the natural boundaries 
by artificial means are probable. But on the other hand certain species may have 
become extinct, at least in parts of their original range, through human agency. Of 
this we have many instances, but in our state none has gone so far as to entirely 
obscure the original conditions. We have pointed out above that the absence of C. 
limosus in the Delaware and Schuylkill Rivers in the region of the Great Alleghany 
Valley may be due to the pollution of the streams issuing from the anthracite region. 
That these rivers, as well as the Susquehanna are considerably polluted partly by 
city sewage, partly by mine-water, is sure (see Leighton, 1903, p. 112, and 1904, 
p- 48), but whether the absence of C. limosus in this region is due to this fact, or not, 
cannot be settled. 
It is different in the western part of the state. Here CO. obscwrus originally oceu- 
pied all of the Monongahela and Alleghany drainages west of the Chestnut Ridge, 
but there are many streams in which it is now lacking, and in which we must assume 
