146 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



Nests observed by our party varied in dimensions from 

 about 18 inches in length, breadth, and height to three feet 

 in diameter and two or three feet in height, enormous struc- 

 tures when it is remembered that the maximum total length 

 of the mouse is only about seven and one-half inches. 



(4) Skeleton, form and makeup 

 (a) Possible parasitism of Phenacomys longicaudus 



All nests observed by members of the Museum party in the 

 vicinity of Hearst and Lierly's Ranch were apparently the 

 appropriated nests of Schinis grisens griseus, many of the 

 sticks used in their construction being too large to have been 

 carried by the tree mice. Shelton reports that he has found, 

 in the course of his investigations in Oregon, only one nest 

 of original Phenacomys construction, all others being old and 

 remodeled nests of the gray squirrel. 



All nests examined by us at Mendocino City, however, were 

 probably built by tree mice exclusively, since there were no 

 sticks used which were too large to be handled by them alone. 



An interesting question arises as to the occupation of the 

 gray squirrel's nests by Phenacomys. Does the tree mouse 

 parasitize and finally drive out his larger arboreal neighbor, 

 or does the mouse appropriate the nest only after its abandon- 

 ment by the gray squirrel? 



(b) Form and composition of the nests 

 Although one's first impression of those nests studied at 

 Mendocino City was that they were loosely built, it was soon 

 discovered that they were steadily fastened and rather diffi- 

 cult to dissect. Usually the structure was spherical in form 

 with a slight flattening on top. Often the flattening was so 

 pronounced as to make the nest hemispherical in form. The 

 skeleton of the nests was formed entirely from twigs of the 

 grand fir (Abies grandis). As used in the nest mass these 

 twigs were dry and leafless. Making up a close interpacking 

 was much material composed of the net-like fibrous mat 

 of the net lichen, RamaUna reticulata, and of the resin ducts of 

 fir leaves, the latter material predominating. Piled up above 

 the thick mass of the main nest there was always a loose 

 superstructure of twigs and branchlets. Some of these were 

 of considerable size for the small mouse, the largest being 

 one-eighth inch in diameter and four to eight inches long. 



