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WOODWARD HIPPURITIDiE.
43
and empty, like the cellular structure between the laminae of certain
Oysters.
Figs. 4 & 5. — Upper and lower valves q/" Hippurites radiosus, Desm.
Fig. 4.
Fig. 5.
Fig.
Fig.
4. Interior of lower valve (^).
5. Model of upper valve : a, a', adductor impressions and processes ; c, c, car-
tilage-pits ; t, f, teeth and dental sockets ; u, umbonal cavity; p, orifices
of canals ; I, ligamental inflection ; m, muscular, n, siphonal inflections.
The umbonal cavity of the lower valve is contracted by three
ridges, produced by inflections of the outer wall ; they correspond to
the three furrows on the outside. The first, or ligamental, inflec-
tion is very slight, and opposite to the centre of the hinge, which
consists of two deep dental sockets, divided by a tooth-like process,
and separated from the shell-wall by two narrow and deep pits for
the internal ligament .cartilage) . In front of the hinge is a large
muscular impression consisting of two portions, and answering to the
anterior adductor, which usually consists of two elements in ordinary
bivalves. Behind the hinge, and between it and the second inflec-
tion, is a deep pit, marked inside with the impression of the posterior
adductor muscle. This inflection, therefore, appears to represent
the lamina which supports the posterior adductor in the fossil genus
Dicer as, and in the recent Cardilia.
The third inflection may possibly correspond to the ridge which
indicates the division of the siphonal orifices in some bivalves, such
as the LedcE and Trigonia aliformis.
In H. cornu-vaccinum the form of the interior is rather diff'erent
(fig. 3) *. See also PI. IV. figs. 2 and 3. The ligamental inflection
is very deep, and the dental sockets are placed across the interior of
the shell, instead of parallel with the side, leaving a shallow cavity
in front, which may perhaps have lodged the internal ligament, for
there are no distinct ligamental pits in the lower valve of this species.
Hippurites which have lost their inner layer exhibit either two or three
longitudinal ridges, accordingly as they belong to the same division
with H. cornu-vaccinum, or with H. radiosus and bi-oculatus.
The interior of the upper valve of the Hippurite appears not

• Compare D'Orbigny's figure, Paleont. Fran^aise, Ter. Cret. pi. 527. f. 2.