one third or one fourth of their own length from the apex of
the carina, rarely by their entire length.

[39] These have also been observed by Dr. Coates; see 'Journal of
Acad. Nat. Sci. Philadelphia,' vol. vi, p. 134, 1829.

_Carina_ (fig. 2 _a_), lying nearly parallel to the scuta, concave
within, very slightly bowed, of nearly the same width throughout, but
with the lower third beneath the umbo, generally a trace wider than the
upper part. Length, variable, generally rather longer (sometimes by even
one third of its own length) than the scuta, but sometimes equalling
only three fourths of the length of the scuta; generally longer than the
terga. Upper and lower points rounded; in rare varieties, both ends are
sharply acuminated. The carina and terga are generally most acuminated
where they are smallest and least perfectly calcified; and consequently,
in this same state, the valves stand furthest apart.

_Peduncle_, flattened, gradually widening as it joins the capitulum, to
which it is generally about equal in length, or a little longer.

_Filamentary Appendages._--Six on each side (Pl. IX, fig. 4), of which
one (_h_) is seated on the posterior margin of a swelling, beneath the
basal articulation of the first cirrus, and this is the longest; the
second (_g_) is short and thick, and is seated a little lower on the
side of the prosoma, (near to this, there are also two little pap-like
eminences;) the third (_i_) is seated on the posterior margin of the
pedicel of the first cirrus, above the basal articulation; the fourth,
fifth, and sixth (_j_, _k_, _l_) in similar positions on the pedicels of
the third, fourth, and fifth cirri. These three latter filaments are
shorter and smaller than the first three. At the base of the second
cirrus, which has no proper filament, there is a swelling as if one had
been united to it.

_Mouth._--_Mandibles_, with the basal edges of the five teeth pectinated
by minute, short, strong spines on one side; inferior angle extremely
short. In one specimen, there was a minute pectinated tooth between the
first and second; in another, the second tooth was bifid on its summit;
in another, the fourth was rudimentary.

_Maxillae_, with five steps: sometimes each step commences with a spine
rather larger than the others; at the upper angle, there are two large
unequal spines (neither pectinated,) with a third longer and thinner,
seated a little below. _Outer maxillae_ (Pl. X, fig. 16), s