31 
the two grooves in question more forcibly impressed. In Fissilunula, ® and 
]f'^ are hardly, if at all, perceptible, and the third becomes much modified. 
It j)asses almost transversely across from the nmhonal apex, and more or less 
bisects its single projecting cardinal tooth a, hut in I. liumana, what I believe 
to be the homologous groove (f (PI, XI, Pig. 3) curves from underneath the 
inrolled umbo, and follows, hut is distinct from, the ligament groove //^ 
and reaches the edge of the articulus immediately over the anterior end of 
the superior horizontal cardinal tooth. I cannot divest myself of the belief 
that a thin lamina of the ligament in /. humana extends beyond the anterior 
end of the nymph in each valve round and under its inrolled beak, bounded 
above by the ligament groove Jr (PI. XT, Pig. 3), and below by the 
impression (f (PI. XI, Pig. 3) just described. 
To return to the transverse groove g (PI. X, Pig. 2) in Fissikmtila, 
what is its nature, and of what is it significant ? Between it and the edge of 
the nymph a triangular space 7ti (PI. X, Pig. 2), is described, bearing ridges 
and grooves coincident n ith its lower concave margin. This presents precisely 
the ajipearance of a space prepared for the reception of an elastic link between 
the valves, and such I believe to have been its nature. If, therefore, I am 
correct in my interpretation of the ligamentary structure of I. humana^ we 
have the explanation of the space m (PI. X, Pig. 2), in Fissilunula^ but 
carried to a greater degree of development in the latter; the ligament there- 
fore will be amphidctic. 
I believe it is also possible to honiologise the dental characters of 
Fissilimula on the one hand with those of I. liumana and I. lumdata on the 
other. It will render a comjiarison clearer if the articuli of I. liumana and 
/, lunulala are first compared, and here attention may be at once called to the 
much tighter spiral, and hoAV much more contracted the jiarts are in the latter 
species than in the former. Priinaiily, it may be noted that the sujieriorand 
inferior cardinal teeth a^ and (PI. X, Pig. 3) in I. lumilata are more 
nearly above and below one another, with relation to the apex of the valve, 
than they are in I. liumana, a^ and ir'^ (PI. XI, Pig. 3) ; secondly, the socket 
F (PI. X, Pig. 3) for the recejotion of the divided cardinal of the left valve is 
more contracted, and extends less in an anterior direction in I. lunulata than 
it does in I. liumana, F \ thirdly, the nymph f '^ (PI. X, Pig. 3) is far more 
contracted from above downwards in the first of these forms than it is in the 
second,/"; fourthly, the accessory ligamental groove (f (PI. P, Pig. 3), in 7. 
lunulata, ajipears to be more highly developed and deeper than in I. liumana, 
