424 MEMOIRS OF THE CARNEGIE MUSEUM 
in the month of November (in the neighborhood of Pittsburgh), after the first kill- 
ing frosts.” 
It is evident from the foregoing observations and considerations that the “ chim- 
neys” are not necessary parts of the burrows. They are simply the result of the 
work of the crawfish, and only represent the material removed from the holes, 
which must be carried somewhere, and is most conveniently disposed of right at 
and around the mouth of the hole. The regular shape of the chimneys is simply 
due to the way the crawfish has to work under certain conditions, and to the phys- 
ical properties of the clay. 
Nevertheless there are certain advantages connected with the shape of the chim- 
neys, which, however, are by no means always present, and, in my opinion, are not 
originally intended. The uppermost part of the hole generally has the tendency 
to be more or less vertical; an addition of 4 to 12 inches adds so much to the 
length of the vertical canal, and the crawfish, when sitting at or near the mouth, is 
able to suddenly drop down to escape enemies, which, as I have repeatedly 
observed, it actually does. Thus a considerable length of the vertical part is 
decidedly advantageous, giving the crawfish a chance to get more suddenly and 
effectually out of reach of danger. 
Another effect of the mud-pile is noticed when the hole opens horizontally on 
sloping ground (banks of ditches). Here the mud-pile generally is semicircular, 
convex toward the ditch, concave toward the mouth of the hole, and thus serves to 
keep the water at a uniform level in the hole (Pl. XLI, Figs. 2 and 3), for generally 
in such cases the hole has spring-water flowing out of it. This may be advantage- 
ous under certain conditions, since I have often found that by removing a pile of 
mud of this character I was able to drain off the water from a considerable part of 
the hole, thus making it distinctly uncomfortable for the crawfish, as is evidenced 
by the fact that it often came out of the hole, as if to investigate the cause of the 
sudden disappearance of the water. 
89In 1905 it began rather late. On November 8, in Fern Hollow and Nine-Mile Run, no fresh chimneys were 
seen on the golf links, where C. diogenes is abundant. On November 22 fresh mud-piles were numerous at the ident- 
ical locality, several sharp frosts having occurred in the meantime. The same dates and the same facts were ascertained 
for C. monongalensis in Fern Hollow. Mr. Kelly’s observations were made November 14 and 15, 1904, but in 1904 I 
noticed fall activity as early as November 5 (Nine-Mile Run, ©. diogenes). (See Pl. XLI, Fig. 5.) 
