92 
with a dilated but slightly impressed contraction in the middle ; 
epidermis transversely wrinkled^ iight-browii, the raised oblique 
portion of the shell yellowish-white ; cortex with membraneous 
scales and filaments^ and covering all the anterior portion of the 
shell to the beak* 
Modiola Americana ^ 'Leach. Zool. Misc.^ vol. 2^ pL 1. 
Var. 
Inhabits the Southern coast. Length from the tip of the angle 
to the middle of the base^ three-fifths of an inch nearly. Breadth 
one and one-fifth of an inch. Thickness half an inch. Cabinet of 
the Academy and Philadelphia Museum. 
This variety ditfers from that figured by Dr. Leach^ in being 
always destitute of oblique colored radii. 
Modiola castanea. — Shell transversely oblong suboval; hinge 
margin elevated in a right line from the beak to the alated angle^ 
from which it declines in a line slightly arquated ; anterior margin 
rounded at the tip ; posterior margin rather large ; base with a 
dilated but slightly impressed contraction before the middle ; epi- 
dermis concentrically wrinkled^ castaneous ; cortex not continued 
behind the middle of the shell ; within bluish. 
Inhabits the Southern coast. Lengthy from the tip of the angle 
to the middle of the base^ three-fifths of an inch. Breadth one 
and one-tenth of an inch. Thickness nearly half an inch. Cabi- 
net of the Academy and Philadelphia Museum. 
This species^ like the preceding, is furnished with a inembrana* 
ceous expansion over a portion of the epidermis, giving rise to a 
number of filamentous processes, by means of which the shell is 
attached to various marine bodies. 
Auca ponderosa. — Shell somewhat oblique, very thick and 
ponderous, with from twenty-five to twenty-eight ribs, each marked 
by an impressed line ; interstitial spaces equal to the width of the 
ribs ; umbones very prominent ; apices remote from each other, 
and opposite to the middle of the hinge, spaces between them with 
longitudinal lines as prominent as their corresponding teeth ; ante- 
rior margin cordate, flattened, distinguished from the disk by an 
abrupt angular ridge ; posterior edge rounded, very short ; inferior 
edge nearly rectilinear, or contracted in the middle. 
Length two inches and one-eighth. Greatest breadth two inches 
and a half nearly. Inhabits the Southern coast. My cabinet. 
