
No. 425.] ANADONTA GRANDIS SAY. 397 
along this line produced, from the umbo to its point of inter- 
section with the shell margin. The proportions then are: 
umbo to anterior adductor umbo to posterior adductor 
: prod E 
umbo to anterior margin umbo to posterior margin 


The umbo in this species comes to a sharp point, which was 
in no case eroded. It afforded a definite point from which to 
measure. The measurements were made along the outer sur- 
face of the shell. The shells were thin and quite transparent, 
so that lines and points marked on the inside were readily 
traced by a soft pencil on the outside by holding the shell to 
a strong light (Fig. 4). Most of the shells were four years old, 
a few three, and some two. (The exteriors of two- and four- 
year-old shells are given in the figures) Measurements were 
made and the ratios calculated for two hundred and seventy 
anterior adductor scars, and for the same number of posterior 
adductor scars. 
In order to plot a curve for these ratios, the following classes 
were taken for the anterior adductor : 
CLASSES. FREQUENCIES. 
585 — .604 
605 — .624 8 
625 — .644 27 
645 — -664 38 
665 — .684 9o 
685 — .704 64 
705 - -724 27 
725 — -744 13 
745 - -764 ee 
270 
The classes for the ratios of the posterior adductor are: 
CLASSES. FREQUENCIES. 
575 — -594 
595 —-614 8 
615 — -634 25 
635 — -654 71 
655 — -674 96 
675 — -694 45 
695 — -714 19 
415 —.734 4 
335 — -754 a 
