202 BULLETIN OF THE 
species; and H. (Celocentrum) irregulare of Gabb, from the high table-lands back of 
Mulege, in the peninsula of Lower California. All of these are separable at a 
glance from Arizonensis. 
The above is Stearns’s description and figure from Proc. U. S. National Mus., 
Vol. XIII. p. 208, Plate XV. Figs, 2, 3, 1890. 
Onchidella borealis, Daw. 
Coos Bay, Oregon. 
It is gregarious in its habits. Fifty specimens were taken in a small crevice of 
clay shale, near high tide. Single individuals, or several clustering together, were 
taken afterwards lower down on the tide under loose stones. When in motion, the 
animal moves off quite rapidly for so small a creature, with two short, stout pedun- 
cles protruding in front of the mantle, bearing keen, sharp black eyes. The color is 
dark slate, splashed with blotches and streaks of ashen white. The body when in 
motion is 4 inch long, 33; wide, } high, and oblong-oval in form, a little broader 
behind than before. It is covered with small tubercles, which are larger around 
the edge of the mantle than those higher up on the body, giving the edge of the 
mantle a serrated or tooth-like appearance when the animal is at rest. When 
it is at rest on a smooth surface, the base of the animal is nearly circular, 
or a little longer than wide, the centre of the body is elevated to quite a sharp 
apex, which together with its color resembles some varieties of a very young 
Acmea pelta, and would be very readily taken for such by an inexperienced col- 
lector. The foot is white, and works in rapid undulations when the animal is in 
motion. 
The above remarks are made by Mr. Hemphill in a recent letter. 
