A Deceptive Fossilizatioii. — Sardeson. 43 
varying degree to that of the common black imprint. Besides 
the above there occur at Minneapohs, casts of M. plana H. 
in shale strata above the limestones, and these were probably 
from clay-filled shells, the filling having gained firmness by 
infiltration of lime from the shell, and the casts are thus often 
normal. The shells have either disappeared or are represented 
by a thin covering upon the cast, with intensified concentric 
varices of errowth. 
The most favorable form of preservation for study of the 
species is that of the uncompressed casts and molds in lime- 
stone. These show, as expected, characters not heretofore de- 
scribed in the species, especially the hinge plate is different 
from that which has been supposed to exist. 
It seems quite unnecessary to repeat here a detailed description 
of the shell's outline which is represented bj^ the accompartying figure, 
but it may be added that the shell varies somewhat in curvature of 
the ventral margin, a slight sulcus being evident rarely. Also in 
the posterior extension of the cardinal line, young shells have gen- 
erally acutely roKuded angles, while larger ones may have the same 
broadly rounded. A broad umbonal ridge extends obliquely backward 
and above it, the shell is concave, giving a decidedly alate appearance 
to the postero-cardinal margin. Anterior to the beaks is a similar but 
small alate flattening. The beaks curve, approaching each other and 
the cardinal margin. Varices of growth are few to numerous on 
the surface of the shell 
Internal scars show the pallial line far from the shell margin, 
especially so in larger specimens. The anterior muscle scar is rcni- 
form. deep, and crossed or "divided" by a small ridge, which appears 
as a depression on the cast. Above the anterior adductor scar is -i 
small, deep protractor scar. The posterior adductor scar is large but 
not deeplv impressed. The shell thickness was i to 0.5mm. in ,in 
average specimen, and there was a broad strong hinge pk.te, which 
beneath the beaks, bore one large broad tooth on each valve with 
corresponding sockets, the tooth on the left valve bemg anterior to 
that of the right valve. Four specimens show this structure, two of 
them by cast of the edge of the cardinal plate as in fig. 6. and two 
others by the whole surface, one of each valve. The degree of 
individual variation can not be determined from so few specimens, 
but they indicate that a strict interpretaion from a single specimen 
would be erroneous for the species as a whole, since there are. various 
additions to the one tooth, viz. a small posterior tooth on the right 
valve of one specimen, a small left anterior tooth on another specimen, 
and a slight tooth or sub-fold upon the contact face of the large teeth 
in a third. 
The species varies in all characters, but not evidently to 
such a degree that absence of hince teeth is to be expected 
