est seen in the
growth-ridges) a little towards the exterior keel. On each side of the
central exterior keel, there is a narrow longitudinal ridge,
corresponding with the interspace between the sub-carina and the
next-but-one latus of the lower whorl; the latus next to the sub-carina
is very small, and overlies the ridge itself. In a very large specimen,
these lateral longitudinal ridges formed (as they likewise did on the
rostrum) slight prominences on the basal margin. In one specimen the
carina was straight.

_Rostrum_ closely similar, in almost every respect, to the carina, even
to the exterior, lateral, longitudinal ridges, and in their relation to
the interspaces in the lower whorl. The valve is generally not so long,
but rather wider, more inwardly bowed, and with the exterior solid keel
less prominent than in the carina. The inner growing surface (fig. 3
_b'_ _d_) is less acuminated at its upper end.

_Upper pair of Latera._--These are much larger than the remaining valves
of the lower whorl; they are straight, triangular, and much acuminated,
with their apices, when well preserved, extending far up, for fully
three fourths of the height of the scuta. They nearly equal in length
the carina. The growing surface (fig. 3 _b'_, _a_) is flat, triangular,
in well-preserved specimens forming only a third or a quarter of the
entire length of the valve. In the middle of the basal margin there is a
very slight prominence, corresponding with a slight external central
ridge, formed as heretofore by the overlapping of two of the valves of
the lower whorl. Basal margin nearly on a level with that of the scuta
and with the basal points of the terga. The foregoing eight larger
valves form the main cavity, in which the body of the animal is lodged.

_Valves of the Lower Whorl._--These, seen externally, seem to belong to
more than one whorl, but internally their basal margins stand on a
level. They vary in number, as already stated, from 22 to 26. I have
seen an individual with a valve more on one side than on the other. They
are of unequal sizes, but they are rather variable in this respect: the
largest are not above half the size of the upper latera: three or four
pairs, together with the sub-rostrum (_e_) and sub-carina (_c_), are
always larger than the others: these two latter valves differ from the
others only in being more concave. Seen externally, all these valves
project considerably, and curl a little inwards, with the