168 MAZATLAN BIVALVES 
Shell extremely thin; lower valve white, smooth; upper 
whitish, more or less stained with golden yellow, rarely salmon 
coloured, extremely rarely greenish, most rarely with a slight 
touch of pink ; smooth, generally developing irregular radia- 
ting ‘ribs near the margin, sometimes strong ones all over. 
Occasionally concentric waves of growth are developed. Lower 
valve dead: white within near the somewhat large (sometimes 
small) hole, gradually becoming transparent; upper valve 
with a defined white spathula, somewhat in the shape of an 
isosceles triangle with an excurved base, radiating from the 
light green ligament, and enclosing the scars. The plug scar 
is not much larger than the others, and is rounded subquadrate. 
The others are round, slightly separated, and at a variable 
angle according to the shape and age of the shell. In different 
specimens examined, the lines joining the centres of the ter- 
minal sears with the centre of the middle one, are inclined at 
angles varying from 105°—133° ; shewing how necessary it is 
to be cautious in founding species merely on the position of 
the sears. If this divergence of central angle were given in 
descriptions, it would greatly aid the student. The subcardi- 
nal sear is small and very slightly impressed. There is a short 
support from the hinge fulcrum in the lower valve. The plugs, 
of which a few were fortunately preserved on the outer valves, . 
are very different in appearance from those of A. ephippium ; 
being large and spreading at the base, but short and small in 
the attached portion. Extremely fine lines run down the raised 
part to the margin of the base, but the rest of the base is not 
grooved. As the shell grows older, it appears to increase the 
raised part near the umbo, withdrawing from the grooved part 
of the base, which is no longer covered by the hole. The hole 
is very much larger than the raised part, which fills only the 
middle of it. One plug has a large tail, like the expanded foot 
of a Cyclas, from the gradual shifting of the animal. The raised 
part has generally a horse-shoe extremity, sometimes built up 
with very loose shelly matter. The plug under the microscope 
appears essentially shelly, though with much animal matter. 
Very young shells are generally orbicular, beginning life as an 
irregular body without prominent apex, shaped like Discina. 
The smallest valve is ‘09 across ; the largest specimen measures 
long. 1°9, lat. 2°44, alt. ° 14. 
Hah.—California Lady Wigram, B. M.—Monterey, 60 fm., Ma- 
jor Rich—lWLa Paz, do.— Panama; at low water mark ; 1 sp.: 
_ OC. B. Adams.—Mazatlan : common and very fine ; Z’pool Col. 
