tends up to the apex; it is of variable depth. Internally the scuta
are concave, and in some old specimens to a high degree. In these
latter, the basal margin, towards the tergo-lateral side, is strongly
sinuous; the prominences are formed by the terminations of the external
longitudinal ridges, and correspond to the interspaces between the
valves of the lower whorl. These ridges, which are interesting, from
throwing light on similar ridges in some fossil species, are present,
both on old and young specimens, and run from the apex of the valve, in
a slightly curved line, to the tergo-lateral half of the basal margin,
where, as we have just seen, they sometimes form prominences. They
consist of three or even four obscure, almost confluent, ridges, of
which the middle one is generally (but not always) the smallest:
together they cover the whole of that part of the scutum, which is not
overlapped along the basal margin by the rostrum and large upper latus;
and they seem evidently due to the growth of the shell in this
interspace having been freer. So, again, the three or four small,
confluent, component ridges have the same relation to the interspaces
between the small latera of the lower whorl.

_Terga_ large, four-sided, with the internal growing surface (fig. 3
_a'_ _b_), almost diamond-shaped; basal angle blunt, rounded;
exteriorly, from the apex to the basal angle there is a rather broad,
very slight prominence, which bears the same relation to the carina and
upper latus, as do the compound ridges on the scuta to the rostrum and
upper latus. The upper part of the scutal margin forms a
slightly-projecting, rounded shoulder, though variable in its degree of
prominence, in relation to the variable depth of the recipient furrow in
the scuta. Externally, parallel to the occludent margin, and close
below the prominent shoulder, just mentioned, there is a slight and
variable depression, extending up to the apex of the valve. This
depression is due to the prominence, variable in degree, of the tergal
edge of the recipient furrow in the scuta.

_Carina_, triangular, strong, inwardly bowed, generally with a large
upper portion freely projecting; exteriorly with a narrow, sharp,
central ridge or keel, which is solid, the interior concavity not
reaching so deep; inner growing surface (fig. 3 _b'_, _b_) deeply
concave, triangular. Basal margin square--that is, transverse to the
longer axis of the carina, or it even rises (as is b