OBSERVATION’S ON THE LIFE-HISTORY OF THE COMMON CLAM. 
195 
In drawing a great many outlines with a camera, two individuals of the same 
length very frequently presented differences in outline which were considerable. 
Everyone has probably noticed how great are these variations in form in the shells 
of the adult ciams, even where the shells have not become distorted in growth by 
coming in contact with unyielding bodies, such as embedded stones. The outlines 
selected and here reproduced are, of course, representative, and show one or two 
curious facts which would appear in any similar series. The first of these is that the 
small rounded shell, as already described, becomes relatively much elongated. Again, 
in the shell 0.4 mm. in length, the umbo appears near the middle of the shell, and 
then rapidly shifts its position anteriorly, as the creature becomes older. In outlines 
9 aud 10 in the series (in individuals 6 ami 7.5 mm. in length), the umbones are being 
gradually moved back toward the middle of the shell, and this is continued in older 
shells until, as in the adult, they have again assumed a position about equally distant 
from the anterior and posterior extremities. This shifting in the position of the 
umbones is of course due to the fact that the shell for a time grows more rapidly 
posteriorly, aud at a later period the anterior part has a more rapid growth. 
Fig. 2. My a arenaria , with shell 0.4mm. long, removed 
from attachment to seaweed (Enteromorpha) and 
showing the single, branched byssus-tliread (b) aris. 
ing from abyssus gland at base of foot (/). The filmy 
siphons ( s ) are shown protracted. 
Fig. 3. My a arenaria. Form 2.3 mm. long (drawn on 
smaller scale than fig. 2), removed from burrow in 
sand, and showing attachment of byssus (b) to 
numerous sand grains (s.y.). 
In shells not longer than 2 mm., it is not difficult to detect the usual tooth in the 
left valve (as well as the excavation in the right), which Gould & Binuey describe in 
the adult as erect, “rounded at its summit, of about equal breadth aud height; its inner 
face is smooth and rounded; its outer face is divided into two portions, the largest of 
which is spoon-shaped, the other tiat, and traversed across the middle by a grooved 
ridge, which projects beyond the margin of the tooth like a smaller tooth.” This 
description may be easily applied to the small shell. In the smallest forms examined 
there was a concrescence of the mantle folds similar to the condition in the adult. 
