NATURAL SCIENCES OF PHILADELPHIA. 121 



Achelous gibbsii. 

 Achelous depressifrons. 

 Arcnseus cribrarius, Dana. 



Sesaima reticulata, Say. 

 Sesaima cinerea, Say. 

 Gelasimus pugilator, Latr. 

 Gelasimus minaz, Lee. 

 Gelasimus pugnax, Smith. 



The fiddler crabs occur in immense numbers throughout the 

 marsh, and the muddier banks of the harbor. The three species 

 given above were taken together, and appear to associate indis- 

 criminately. They form a considerable part of the food of vari- 

 ous birds, especially the herons, rails, gulls, fishcrows, and grackles, 

 and many are destroyed by the larger crabs, as the Stone, and 

 the common edible species. They may be found at all seasons, 

 but are much more numerous during the warmer months than in 

 winter, when, however, some are seen abroad ; and they may 

 always be taken by digging into their holes. Of their numbers 

 one who has not seen them can form no adequate estimate : they 

 gather about decaying substances until the ground for several 

 square j^ards may be completely covered; and troops running 

 throuoh the scant herbage of the marsh make a rustlinc; sound 

 They doubtless exercise great influence upon the natural economy 

 of the localities they frequent, in devouring decomposing matters, 

 both vegetable and animal. They swim with ease, but in general 

 avoid the water, and appear to prefer rather dry and sandy places 

 to the heaviest mud. This preference is especially marked in the 

 selection of places for boring the holes ; this is done chiefly, I 

 think, during the night, at least I do not remember to have seen 

 the animals so engageel ; and frequently places I had walked over 

 the day before, when there had been no holes seen, were riddled 

 through in the morning. The holes average in diameter little 

 over half an inch, and look exactly as if launched by the end of 

 a walking-stick ; they are sunk at all angles with the surface, are 

 oftener tortuous than straight, and generally a foot, or less, deep. 

 Little piles of sand near the entrance show the fresher excava- 

 tions, as they are soon levelled b}^ natural causes. The tunnels are 



1871.] PART II.— 9 



