1895. } NEW YORK ACADEMY OF SCIENCES. 119 
“oinal area for the protection ofa mantle and setz can be seen. 
The outer posterior angles of the embryonic shell were turned 
upward, giving it somewhat of a saddle-shaped relief (fig. 1d). 
In some examples the shell is crossed by a raised band due to 
the strengthening of the hinge line during the subsequent growth 
of the shell (fig. ‘la). 
Already, at this very early period, we find clearly, though 
very minutely displayed, the rasp-like surface, which is so marked 
a feature of this species during the next period of its growth. 
(2.) In the second phase of growth there is a decided change 
in the form cf the shell. There was also an extension of the 
hinge line and a transfer outward of the muscles along that line 
to accommodate the growth of the animal; but although the 
hinge line is actually much longer than in the embr yonic shell, 
owing to the more rapid enlargement of the sides and front, it 
appears to be shorter. This we may regard as the larval or 
‘neepionic stage of the shell, as by its form and features it ex- 
hibits indications of the possession of organs for the capture of 
food (fig. 1 /). 
In this stage of its growth the overlapping outer layers of 
the shell are ridged up around the posterior angles at the ends 
of the hinge line of the embryonic shell, showing that now the 
agency of the mantle was exercised in adding the margins. In 
this stage of growth the shell was still quite thin, and the 
several stages of growth are clearly marked by the impression 
of the gradually enlarg? ng previsceral depression, now less dis- 
tinctly ‘indented than in the embryonic shell; we can trace the 
growth of the shell by the anterior and posterior points of the 
previsceral depression, as well as by the two series of scars, 
diverging from the posterior margin that mark the periodical 
change i in the position of the anterior adjustor muscles (figs. 1 tg 
and q). 
Owing to the thinness of the shell at this early period of 
growth, the frequent enlargements of the shell are shown by the 
defined margin of each shell layer; of these about six can be 
distinguished on the inner half of this zone. The surface of 
the shell in this part bears the rasp-like ornamentation charac- 
teristic of the species on all except the last one or two layers, 
where the growth is indicated only by ridges concentric to the 
umbo, and “here there is no radulated surface. The rasp-like 
ornament on these shells is not unlike that on Acrothele; but 
in Acrothele the markings are more distinct near the margin 
than toward the umbo, while the reverse is the case with this 
shell. 
This larval or nepionic part of the shell is sometimes divided 
