On May 13, 1883, I chanced to enter a meadow a few miles above Washington, on the Virginia side of the Potomac, at the head of a small stream emptying into the river. It was between two hills, at an elevation of 100 feet above the Potomac, and about a mile from the river. Here I saw many clayey mounds covering burrows scattered over the ground irregularly both upon the banks of the stream and in the adjacent meadow, even as far as ten yards from the bed of the brook. My curiosity was aroused, and I explored several of the holes, finding in each a good-sized crayfish, which Prof. Walter Faxon identified as _Cambarus diogenes_, Girard _(C. obesus_, Hagen), otherwise known as the burrowing crayfish. I afterward visited the locality several times, collecting specimens of the mounds and crayfishes, which are now in the United States National Museum, and making observations. At that time of the year the stream was receding, and the meadow was beginning to dry. At a period not over a month previous, the meadows, at least as far from the stream as the burrows were found, had been covered with water. Those burrows near the stream were less than six inches deep, and there was a gradual increase in depth as the distance from the stream became greater. Moreover, the holes farthest from the stream were in nearly every case covered by a mound, while those nearer had either a very small chimney or none at all, and subsequent visits proved that at that time of year the mounds were just being constructed, for each time I revisited the place the mounds were more numerous. [Illustration: Fig. 1 Section of Crayfish burrow]