PELECYPODS 419 
muscle scars) was, in life, covered by the mantle, which adhered 
tightly to it. The space between this pallial line and the outer 
edge of the valve was occupied in life by the free portion of the 
mantle. Frequently a depression in the pallial line toward the 
center of the valve is seen in the posterior end. This is called 
the pallial sinus, and marks the space occupied by the siphons. 
The pallial sinus is absent, moderate, or deep, according as the 
animal possessed no siphons, or small or large ones. 
The length of bivalve shells is measured from their anterior to 
their posterior ends, while the height indicates the greatest diam- 
eter between the umbones and the ventral margin. 
The greatest possible degree of variation exists in the structure 
of the pelecypod shell. Every degree of thickness from the most 
fragile, tissue-like structure to the very heavy and ponderous 
tests of some of the Veneride is to be found. There is also a 
range in size from the very minute Spheria to the giant Tridacna 
of Eastern seas, which weighs several hundred pounds. As in the 
Gasteropoda, there is fortunately a very close parallelism between 
shell and anatomical variations. One very quickly learns to 
place a bivalve in its systematic position by a mere glance at the 
shell alone. While the higher classification into orders, sub- 
orders, ete., is based upon anatomical features, the genera often 
and the species always are founded upon conchological or shell 
characters. 
The Pelecypoda offer one of the most interesting fields for — 
investigation and study among the invertebrates. Although by 
no means neglected, they have not received as much attention on 
the part of biologists as has been given to other phyla, or indeed 
to the other classes of this same phylum. 
CLASSIFICATION OF THE PELECYPODA 
Nearly every systematic writer upon the mollusks has attempted 
to give a good classification of the bivalves, yet none of the results 
is wholly satisfactory. The scheme now generally adopted is 
one based upon modifications of the gills, or branchie, and has 
practically replaced all the older classifications, which depended 
