316 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
the conical apex becomes central, though in most cases slightly eccen- 
tric; then the posterior growth of the shell is far in excess of the 
anterior growth and the apex finally occupies the anterior third of the 
shell in its approach to maturity. In these stages the veliger shell 
usually turns to the left and for some time overhangs the posterior 
margin of the permanent shell-growth (Figs. 6 and 7). I have not yet 
succeeded in finding the young of Acmaea alveus before losing its 
embryo shell. The cicatrix on the apex of the smaller specimens of A . 
alveus resembles so closely a similar cicatrix on the young of A. tes- 
tudinalis that I am convinced the embryo shell must be identical in the 
Fig. 6. Fig. 7. 
two species. Dr. Robert T. Jackson (4) was the first investigator 
to figure and describe the cicatrix in A. alveus. He believed, as I did 
at first, that the cicatrix indicated a trace of an early nautiloid shell, 
the round area representing the aperture of the embryo shell as it 
really does, while the lunar cicatrix behind was supposed to show a 
section of the backward coil, or body Avhorl. In both species here 
considered there is considerable individual variation; in A. alveus 
the cicatrix usually shows two parts, a round area distinctly outlined 
which marks the immediate apex of the shell, and, directly behind and 
closely connected with it, a blunt semi-lunar outline, this portion repre- 
senting the position of the Caecum-like embryo shell. In some speci- 
mens this portion of the cicatrix is wanting as if the concentric lines of 
accretion of the permanent shell passed beneath it, leaving the embryo 
shell free and overhanging posteriorly. In .1. iestudinalis the cicatrix 
is precisely the same as in alveus though not so clearly defined nor evenly 
marked, and this is probably due to the different environment of the 
two species — alveus living in cjuiet water on swaying eel grass, while 
testudinalis is found in rock pools at low tide exposed to pounding 
waves. 
