ung
condition they are tubular, but not folded; and often, according to
Prof. Macgillivray, either one or both are at first imperforate. They
are formed externally of the outer membrane of the capitulum (rendered
thin where folded), and internally of a prolongation of the inner tunic
of the sack; between the two, there is, as around the whole sack, a
double layer of corium. A section across both appendages, near their
bases, is given in Pl. III, fig. 4 _b_, showing how they are
folded,--the chief fold being directed from below upwards, with a
smaller fold, not always present, from between the two, outwards. The
folds sometimes do not exactly correspond on opposite sides of the same
individual; they are almost confined to the lower part, the orifice
itself being often simply tubular. These appendages are sometimes very
nearly as long as the whole capitulum: a section near their bases is
sub-triangular. I shall presently make some remarks on their functions
and manner of formation.

The _Scuta_, as well as the other valves, are imperfectly calcified:
shape, variable. They usually consist of two lobes or plates, placed at
above a right angle to each other, and rarely (fig. 4 _c_) almost in a
straight line; the lower lobe is more pointed and narrower than the
upper; the two correspond to the lower and middle lobes in the scuta of
_C. virgata_, the upper one being here absent.

The _Terga_ are developed in an extremely variable degree; they are
often entirely cast off and absent. In very young specimens, they are of
the same length with the carina, but after the carina has ceased to
grow, the terga always increase a little, and sometimes to such a degree
as to be even thirty or forty times as long as carina. When most
developed (fig. 4 _a_) they are not above one third as long as the
scuta, to which they lie at nearly right angles; they consist of
imperfectly calcified plates, square at both ends, slightly broader and
thinner at the end towards the carina, where they are a little curled
inwards, than at the opposite end; they are not quite flat in any one
plane; internally they are slightly concave; finally, I may add, they
nearly resemble in miniature the terga of _C. virgata_. In full grown
specimens, the terga almost invariably drop out and are lost; but even
in this case, a long brownish cleft in the membrane of the capitulum,
marks their former position. The orifice of the capitulum is usually
notched between the terg