950 : THE AMERICAN NATURALIST. [Vor. XXXVI. 
and a quarter of a mile wide, surrounded by woods and corn 
fields. It is traversed by a cool stream, the banks of which 
for some distance on either side are boggy and thickly studded 
with large grass-covered hummocks. The F. cinerea have con- 
structed their formicaries in these hummocks, which range 
from 30 cm. to 60 cm. in diameter at the base and from 20 cm. 
to 30 cm. in height. There are nests in nearly all stages 
of growth, but for the most part well-established and extremely 
populous, being, with the exception of the two nests above 
described, the most populous nests of Formica I have seen during 
the entire summer. The formicary is started in the summit of 
the hummock, but ultimately invades its whole earthy sub- 
stance and extends to a depth of at least 30 cm. to 60 cm. into 
the black soil from which the hummock arises. In small or 
moderately large nests all the grass which originally covered 
‘the hummock remains intact and in excellent condition, but in 
the largest formicaries the grass on the summit is partly 
cut away by the ants and partly buried under the earth brought 
up from the galleries and the little straws, bits of twigs, leaves, 
etc., collected by the insects in obedience to an instinct which 
appears to be shared to a greater or less extent by all the 
species of Formica. This makes the large nests very conspic- 
uous, although the numerous openings, all in the flattened 
or somewhat convex summit of the hummock, are hidden 
under the outermost layer of vegetable débris. The living 
grass forming the sides of the hummock gives the nest great 
stability and very efficiently protects it from being injured 
by the feet of the pasturing cattle. Excavation of larger nests 
shows that the hummocks are honeycombed throughout with a 
network of inosculating galleries abruptly terminating at the 
level of the moist, black meadow soil, into which only a very 
few long and more or less perpendicular galleries and chambers 
penetrate to a depth of 60 cm. and possibly farther.’ 
! During September, after this paper had been sent to the Naturalist, Y 
happened on a fourth locality abounding in cinerea nests. This was 4 large 
meadow almost within the city limits of Rockford. It contained formicaries of 
all three types : under logs and stones, in the form of flat, irregular mounds a: 
in modified hummocks. 
