38 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
The outline is elliptical or subelliptical, with the greatest width usually 
at about the middle of the brachial valve, occasionally considerably 
above or below it. In old individuals the outline is subpentagonal. 
A fold and sinus are present in all except very young individuals but 
this feature also shows variation. In adolescent stages, except the 
youngest, they may be represented by only a slight flexure on the 
anterior margin and in the oldest specimens they are only moderately 
developed. A shallow depressed furrow down the middle of both 
fold and sinus is a constant feature, and appears in the young before 
either the fold or the sinus. 
So much variation occurs in the general shape of the shell that by 
some writers the species would probably be made the basis of at least 
two and possibly more species. All gradations, however, are found 
between extremes of any character. The individuals do not fall into 
certain groups more or less well defined with a few intermediate forms, 
but the whole is a continuous series. There is, further, no associa- 
tion of certain characters; each character varies independently of 
any other. Lastly, the individuals themselves very commonly show 
great irregularity as, for example, one specimen in which the right 
and the left sides of the shell were developed unecpially. The length 
of this specimen is 24.5 millimeters. Had the left side been symmetri- 
cal with the right, the breadth would have been 25.3 millimeters; 
had the right side been symmetrical with the left, it would have been 
30.8 millimeters. 
Unfortunately almost all the specimens occur as internal molds 
with ragged edges and the exact measurement of a large number of 
specimens necessary for biometric work is impossible. If a sufficient 
number of specimens in fit condition for measurement and showing 
the exact outlines of the earlier stages were at hand, it is not impos- 
sible that a plotting of the results would reveal tendencies toward 
development in certain lines, and a division into varieties or even 
species would be advantageous. Under the circumstances, however, 
there would be nothing gained by such a subdivision. This variation 
in external characters cannot be emphasized too strongly because, as 
will be shown presently, it extends to the internal structures as well, 
where differences between indiAdduals occur which some might con- 
sider of specific, if not higher, ordinal value, if found in a type less 
plastic than the one under consideration. 
The outlines on plate G and drawings on plate 8 show the range of 
