120 TRANSACTIONS OF THE [maR. 17, 
“by one or two concentric lines into two parts, of which the 
outer is distinguished by an enlarged pattern of the radular 
ornamentation ; ~and the margin of this zone has a more rounded 
outer margin than the inner zone, owing to the more rapid ex- 
tension of the margin at the sides and front. In this outer half 
of the larval zone there is a more decided thickening of the 
shell, for the lines of growth are not so delicately marked as in 
the inner half, nor is the impression of the features of the in- 
terior of the shell so clearly apparent, except as regards the 
outlines of the visceral cavity. These features, however, may 
be inferred from the rounded ridges on the surface, and from the 
lines of scars left by the anterior adjustor muscles, which give 
evidence of about six stages of growth in this outer zone of the 
larval shell. 
Outside of this zone the radular ornamentation is exchanged, 
on a narrow band of the shell, for concentric ridges, indicating 
an arrest of growth preparatory to the next phase in the life 
history of the individuals of this species. 
The close of the larval or neepionic period is marked by the 
fixation of the hinge line, which no longer lengthens, and con- 
sequently the position of the posterior adductor and the proxi- 
mate end of the cardinal muscle do not materially change after 
this. 
(3) The advent of the next phase in the history of this shell, 
which may be called the adolescent or nealogic phase, is indi- 
cated by a return to the radular ornamentation, which now is of 
a still coarser pattern than previously, and is ‘not always well 
preserved ; in fact dorsal valves are not uncommon, and ventral 
valves are occasionally found, which show no radular ornament 
at this period, but have concentric lines only. This peculiarity, 
however, may be due to imperfect preservation. 
This part of the valve, like the nepionic, is not unfrequently 
found to be divided into two zones by a few concentric lines ; 
in the outer of these zones the radular ornament is usually very 
irregular. 
At this period one does not find the lines of growth so dis- 
tinctly marked as in the earlier period, nor the scars of the 
anterior adjustor and adductor muscles; but the outline of the 
visceral cavity, owing to the thickening of the callus formed 
there, stand out with great distinctness (fig. 2e). 
The adult or ephebolie shell presents, in both valves, a nearly 
round contour, and on it also is rather prominently indicated 
the outline of the visceral cavity, which becomes proportion- 
ately narrower in the adolescent and adult stages, than at an 
earlier period. The marks of the muscles in the previsceral 
