File:The Quarterly journal of the Geological Society of London (12711770344).jpg

PROCEEDINGS OF THE GEOLOGICAL SOCIETY.
to have been figured or described; but Goldfuss and D'Orbigny
have figured a mould of its interior, and I obtained a similar prepa-
ration (fig. 7) by removing the upper valve from one of Mr. Pratt's
specimens with a hammer and chisel.
Figs. 6 & 7. — Upper valve ofH. Toucasianus.
Fig. 7.
Fig. 6. Exterior of the upper valve ; \.
Fig. 7. Mould of the interior of upper valve ; f . I, m, n, duplicatures ; x, fracture,
showing canals ; c, cartilage ; u, left umbo ; — the arrows indicate the
supposed direction of the branchial currents.
This mould shows the umbo turned forwards, and having a deep
furrow on each side, caused by processes from the upper valve. On
the dorsal side of the umbo, close to the ligamental inflection, is a
small conical elevation (omitted in the figures of Goldfuss and
D'Orbigny) representing the cartilage or one of its divisions.
A plaster-cast taken from this mould gives the form of the interior,
to a certain extent ; that is to say, it shows the umbonal cavity, the
cartilage-pit, a deep furrow winding round the adductor and siphonal
inflections, and the bases of the hinge-teeth.
With the help of this mould I filled up the umbonal cavity of the
other specimen (the lower valve of ^. radiosus), and then took from
it a plaster- cast (fig. 5), which gives what I believe to have been the
form of the upper valve with its processes complete.
To test the correctness of this model, I made a number of sections,
both transverse and longitudinal, of Hippurites in which both valves
were preserved. These show that the two prominent hinge-teeth
were extensively under-cut by the umbonal cavity (fig. 8, u), so as
to appear suspended by thin plates. Each tooth supports a process
corresponding in shape to the muscular impressions in the lower
valve ; the anterior projecting horizontally ; the posterior vertical
and tooth-like, longer indeed than the tooth to which it is attached ;
but thinner than in the model, not nearly filling the cavity for its re-
ception (fig. 3, «'). Both these muscular apophyses are under-cut, so
as not to interfere with the channel which winds round the inflections.