220 
MARINE SHELLS OF WEST COAST OF NORTH AMERICA 
as wide as the ridges. These ridges slope abruptly on the dorsal side and 
much more gently on the ventral, the ventral side being about twice as long 
as the dorsal. Of these ridges, which are finely denticulated at their free 
margin, 22 are present in the type. The anterior median area is quite 
narrow and marked by dental ridges bearing rather prominent tubercles. 
These dental ridges join those of the anterior part at almost a right angle. 
The middle median portion forms a depressed groove which is crossed 
by rough U-shaped curved wrinkles. The posterior portion is a little 
wider than the anterior and median portion combined, and is marked 
by a series of wave-like markings, between which finer lines of growth 
are present. The posterior part forms a conspicuous triangular auricle, 
which is rather large and not strongly separated, though somewhat con¬ 
stricted at its junction with the posterior median portion. The interior is 
bluish white. The junction of the anterior and median portions is marked 
by a raised thread. The median portion is a roughened groove at the basal 
termination of which the ventral knob is placed. The posterior part pro¬ 
jects over the posterior median portion as a conspicuous shelf. The free 
auricular portion is marked by concentric lines of growth. The umbo has 
a strong knob, from the basal portion of which a narrow blade extends 
down through about half the length of the shell. Pallets paddle-shaped, 
rounded at the distal end, which is surmounted by two strong periostracal 
prongs, which form a forked tip. The distal half of the outside, which is 
rounded, is covered by a periostracum. The inside is slightly concave, the 
distal half also having a thin brown periostracum. Height, 4.1; length, 
4.1; diameter, 4.1 mm. (Bartsch.) 
Type in U. S. N. M., No. 344665. Type locality, Shaw-Batcher Ship¬ 
yard, South San Francisco, California. 
Range. Known only from the type locality. 
