396 THE AMERICAN NATURALIST. | [Vor. XXXVI. 
to the point where the outer boundary of the anterior adductor 
scar cuts the line, made the first term of the ratio (see xi, x, 
etc., Figs. rand 3). The second term was the distance from the 
umbo along this line produced to its intersection with the shell 




Fic. 1. Pic, 2. 



EI. Fic. 4 

Fic, 1. — Interior of a four-year-old shell, impressi hasized by ink lines. 
Fic, 2. — Exterior of a four-year-old shell. 
Fic, 3. — Interior of a two-year-old shell. 
Fig. 4. — Exterior of a two-year-old shell, 

4;, 42, anterior adductor scars; 7, 3 posterior adductor scars; vr, x3, etc., points on 
ridge between anterior adductor and anteri rior tarator, fixed by its Taleo weg the 
a ductor’s outer margin ; 71, Y2, etc., limit of posterior muscle 
on at which adducto: in i th 

ptt OT E ng to the ridge. 
margin. A similar ridge is formed at the anterior margin of 
the posterior adductor. The first term of the ratio for the 
posterior adductor is the distance from the umbo to the point 
where this line is tangent to the adductor scar (71, yz, etc. 
Figs. 1 and 3). The second term is the distance measured 
