d in many genera, such as _Terebratula_
and _Rhynchonella_, has a prominent beak or umbo, with a circular or
otherwise shaped foramen at or near its extremity, partly bounded by one
or two plates, termed a deltidium. Through the foramen passes a
peduncle, by which the animal is in many species attached to submarine
objects during at least a portion of its existence. Other forms show no
indication of ever having been attached, while some that had been moored
by means of a peduncle during the early portion of their existence have
become detached at a more advanced stage of life, the opening becoming
gradually cicatrized, as is so often seen in _Leptaena rhomboidalis_,
_Orthisina anomala_, &c. Lastly, some species adhere to submarine
objects by a larger or smaller portion of their ventral valve, as is the
case with many forms of _Crania_, _Thecidium_, _Davidsonia_, &c. Some
_Cranias_ are always attached by the whole surface of their lower or
ventral valve, which models itself and fills up all the projections or
depressions existing on either the rock, shell or coral to which it
adhered. These irregularities are likewise, at times, reproduced on the
upper or dorsal valve. Some species of _Strophalosia_ and _Productus_
seem also to have been moored during life to the sandy or muddy bottoms
on which they lived, by the means of tubular spines often of
considerable length. The interior of the shell varies very much
according to families and genera. On the inner surface of both valves
several well-defined muscular, vascular and ovarian impressions are
observable; they form either indentations of greater or less size and
depth, or occur as variously shaped projections. In the _Trimerellidae_,
for example, some of the muscles are attached to a massive or vaulted
platform situated in the medio-longitudinal region of the posterior half
or umbonal portion of both valves. In addition to these, there exists in
the interior of the _dorsal_ valve of some genera a variously modified,
thin, calcified, ribbon-shaped skeleton for the support of the ciliated
arms, and the form of this ribbon serves as one of the chief generic
characters of both recent and extinct forms. This brachial skeleton is
more developed in some genera than in others. In certain forms, as in
_Terebratula_ and _Terebratulina_, it is short and simple, and attached
to a small divided hinge-plate, the two riband-shaped lamina being bent
upwards in the middle (fig. 15). The ca