62 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
half of the shaded portion as it is represented in the diagram. This 
is seen further in the wedge-shaped anterior end of the platform. 
The greater part of the attachment of the adductor is nearer the beaks 
than is usual for Meristelliform genera. When it is now recalled that 
the actual hinge point is at the base of the crura, it is seen that, if the 
adductor muscles were continued beyond the platforms and attached 
directly to the valve, the point of hinging would fall almost directly 
in the line of pull. Attached as they are to the platforms, a consider- 
able mechanical advantage is secured. 
It is not intended to imply that the muscles occupy their present 
position as a direct result of the posterior position of the jugum and 
that the platforms are an adaptive response to the new mechanical 
requirements. Such an explanation would apparently involve a long 
period of accumulation of slight changes, in themselves of little value. 
The platforms of Camarophorella are so fragile that it is impossible 
to imagine that they have ever been called on to withstand any con- 
siderable strain. jNIoreover, the muscle scars themselves are so light 
that the bovmdaries of some have not been determined. This, together 
with the relatively small size of the areas of implantation and the 
closely interlocked hinge, leads to the belief that the musculature 
was below the average — certainly in bulk it was — and that there 
was small demand for the formation of platforms in order to protect 
certain of the visceral organs lying beneath them, as has been assumed 
to explain similar platforms in other genera. This does not mean to 
imply that in ancestral forms they were not developed originally for 
that purpose. 
It seems probable that the condition found in Camarophorella is 
the result of a closer integration of previously existing parts, that the 
shifting of the jugum into the posterior part of the brachidium and the 
concentration of the muscles posteriorly have proceeded together, 
rather than that one has been the cause of the other; it seems, further, 
that both have been made possible by the previous existence of the 
platforms which have permitted the muscles to assume a position 
nearer the beaks than would have been mechanically possible other- 
wise. - 
The relationship of Camarophorella to the subfamily Meristellinae, 
as just outlined, appears to be approximately correct. The peculiar 
type of jugum, the muscle platforms, and the general shape of the 
shell all bear it out. Nevertheless, the study of the jugum has sug- 
