94 
MARINE SHELLS OF WEST COAST OF NORTH AMERICA 
rounded projecting process, fitting into an excavation in the opposite 
valve, which is thickened behind it. Ligament and ossicle moderate. 
Pallial line simple, continuous. Anterior margin evenly rounded; posterior 
ditto, a little more effuse; ventral margin with the faintest possible indica¬ 
tion of flexuosity; shell gaping behind, but with no perceptible ventral gape. 
Exterior smooth except for lines of growth, which are more emphasized 
at intervals, forming three concentric waves from the umbo toward the 
margin. Shell covered with a thin yellowish-brown, pubescent epidermis, 
which is produced over the margins, and under a lens is seen to be very 
closely, finely, radiately striate. The pubescence is formed by little pro¬ 
jecting points of the epidermis at the intersections of the striae with the 
lines of growth. Shell solid. Long., .9; alt., .44; diameter, .33 in. (Dali.) 
Type in U. S. N. M. Type locality, Port Simpson, British Columbia. 
Range. Port Simpson, British Columbia, to San Diego, California. 
Genus MYTILIMERIA Conrad, 1837. 
Shell equivalve, sub-oval, thin, beaks sub-spiral; hinge edentulous, with 
a slight linear cavity under the beaks; muscular impressions two, rather 
small; pallial impression with a broad obtuse sinus. Animal gregarious, 
forming a nest. 
Type. Mytilimeria nuttallii Conrad, 1837. 
Distribution. West coast of United States, in compound Ascidians. 
Mytilimeria nuttallii Conrad, 1837. 
Jour. Acad. Nat. Sci. Phila., 7 :247; pi. 19, fig. 5. 
Shell sub-oval, inflated, thin, fragile; white, a very thin, yellowish, 
deciduous epidermis. Length, 30; height, 31; diameter, 16 mm. (Conrad.) 
Type in U. S. N. M., No. 74234. Type locality, Coast of California. 
Range. Vancouver Island to San Diego, California. 
Family POROMYACIDiE. 
Genus POROMYA Forbes, 1844. 
Shell suborbicular, subequivalve, and inequilateral, thin, transparent, 
slightly nacreous within; valves closed, surface granulated; teeth, in 
right valve a short but strong cardinal, and in the left a minute triangular 
cardinal and a ridge-like lateral on the posterior side. 
Type. Poromya anatinoides Forbes. 
Distribution. Britain, Scandinavia, Mediterranean, tropical America, 
Pacific Northwest America. 
