MILLER : BEACH MOUSE OF MUSKEGET ISLAND. 
87 
teristic of the two species and which I have found remarkably con¬ 
stant in a large series of specimens. Although the skull of the 
3Rcrotus occurring on Nantucket (Plate 1, fig. 2) is as large 
as that of M. breweri , it has the broad, high brain case of M. penn- 
sylvanicus , to which form the Nantucket mouse is clearly referable. 
