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 Veneridce 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, etc., 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 branchiae, and has 

 practically replaced all the older classifications, which depended 



