File:The Quarterly journal of the Geological Society of London (12711339145).jpg

WOODWARD HIPPURITID^.
49
difficulty in comparing the Birostrites (figs. 15, 16) and its "acces-
sory apparatus " .c, c) with the complete Radiolite ; the accessory
apparatus representing the great furrowed cartilage and the dental
processes.
Figs. 15 & 16. — Internal mould of ^. Hceninghausii, Desm.
From the Chalk.
Reduced \.
Fig. 15.
Fig. 16.
Fig. 15. Upper view. Fig. 16. Side view.
M, u, umbo of left valve ; r, right umbo ; /, ligamental groove ; c, c, cartilage ;
a, anterior adductor cavity ; a', a\ posterior adductor.
From the close affinity between the Radiolite and the Hippurite,
we must conclude that the tubular structure in the opercular valve
of the latter possesses less importance, physiologically, than at first
seemed probable.
The presence of canals in the Hippurite and their absence in the
Radiolite is paralleled by the difference in the shell-structure of Te7'e-
bratula and Rhynchonella, and in the test of Cynthia and Ascidium.
Caprotina. — Three other genera, Caprina, Caprotina, and Capri-
nella, are found in the Hippurite-limestone, and are more like Chama
and Diceras in general form. They had an internal ligament, and
were attached by the dextral valve, which is straight, whilst the upper
valve is oblique or spiral.
In Diceras, however deep and spiral the umbonal cavity, there
is no indication of septa ; but they exist in all the Hippuritidce, and
in both valves, whenever the umbones are much produced.
In Caprotina the shell-structure is the same as in Diceras and
Chama ; the outer layer is solid, and consists of corrugated or ob-
scurely prismatic-cellular layers ; whilst the inner layer is sub-
nacreous, and more easily destructible. The lower valve is always
striated or ribbed, the upper plain, as in the fossil Oysters of Barton
and Woolwich .O.Jlabellulum and O. pulchra^ *. The hinge resem-

• Several of M. D'Orbigny's Caprotinoe present no character by which they
  

can be distinguished from Chama ; viz. C. rugosa, C. navis, C. carinata, C. Dela-
rueana, and C. Cenomanensis. The Diceras inaquirostratus, Woodw. 1836, Geol.