424 MEMOIRS OF THE CARNEGIE MUSEUM 



in the month of Novemher (in the neigliborh(M)d of Pittsburgh), after tlie iirst kill- 

 ing frosts.'*^ 



It is evident froni 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 v'ertical ; an addition of 4 to \2 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 [ 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 nuid-pile is noticed when the hole o^jcns horizontally on 

 sloping ground (Ijanks 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 (PI. XLT, 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 fcHind that liy removing a pile of 

 nu)d 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 crawfi.sh, 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. 



^'In 1905 it began rather late. On November 8, in Fern [follow ami Nine-Mile Run, no fresli cbinmevs were 

 seen on the golf links, where C. dliigeiiea is abundant. On November 22 fresh mud piles were numerons at the ident- 

 ical locality, several sharp frosts having occurred in the meantime. The same dates and the same facts were ascertained 

 tor C. mnnongalensis in Fern Hollow. Mr. Kelly's observations were made November 14 and 15, 19(14, but in 1904 I 

 noticed fall activity as early as November 5 (Nine-Mile Run, ('. dingoux). (See PI. XLI, Fig. 5.) 



