16 The American Naturalist. [January, 4 
passage at that area. Such a notch we find in Tridacna, Pecten, 
and many allies. Young Pecten irradians crawls while lying on 
its right valve by extending the foot over the edge of the valve. 
The constant extension of the foot would necessarily cause a 
local retraction of the right mantle lobe at that area; therefore 
shell growth would proceed less rapidly, and a notch would 
consequently be formed. By this action no pressure is exerted 
on the left mantle lobe, and no notch is formed on that side of 
the shell In later life the crawling habit is abandoned, the 
foot atrophied, and the notch nearly or quite disappears, as I have © 
observed in several species of Pecten. The presence of a byssus 
at such an area may induce a notch, as well as the crawling habit, 
as may be observed in Avicula and Meleagrina. Young Hinnites 
and Spondylus are pecteniform and have a deep byssal notch, as 
I have shown;* but as soon as they become attached by cementa- 
tion to a foreign object the use of the foot is of course abandoned, 
and the notch is not perpetuated in succeeding shell growth. In 
the development of Anomia we find that the right valve surrounds 
the byssus completely, enclosing it at an early stage. In later — 
growth the byssus and calcareous byssal plug become greatly — 
enlarged, the walls of the enclosing foramen receding to give — 
space for the enlargement of the organ. This enlargement of 
the foramen is apparently to be explained on the physiological 
principle that constant pressure causes a resorption of tissue. 
In free-crawling or superficially burrowing Pelecypods the foot 
is extended from an area nearly opposite the hinge line, that being 
the most effectual position for crawling while the valves are in an 
upright position. In deep-burrowing forms, as Solen, on the 
other hand, the foot is extended at an area nearer the hinge line 
and in the plane of the longer axis of the shell. It is evident 
that in this position it is more effectual in producing a hole, it is 
in a better position to drag the shell after it, and it offers the least | 
-resistance to the surrounding medium. In deep burrowers, as 
Mya arenaria, and especially Ensatella (Solen) americana, it is to 
Eh INE 
S EAE eS ee 
SE CLASARE EE E n, 
gn Soma illustrating this stage see this journal, December number, page 1138 ` 
® Phylogeny of the Pelecypoda. 
