File:The Quarterly journal of the Geological Society of London (12711499553).jpg

WOODWARD HIPPURITID^. 47
ligamental inflection is seldom indicated externally ; but in those species
which M. D'Orbigny has distinguished as Bi-radiolites (fig. 19)
there is a peculiarly sculptured tract, or " cardinal area," on each
side the ligamental line ; and in R. Fleuriaui the upper valve is
rendered subspiral by a ligamental groove *.
The interior of each valve is nearly symmetrical, but we may infer
from the structure of R. Fleuriaui, that the Radiolite, like the
Hippurite, was attached by the dextral valve ; an inference which
is confirmed by comparing R. polyconilites with Caprotina. The
outer wall or shell-layer of the Radiolite is unlike that of the Hip-
purite. It consists of prismatic-cellular structure like the shell of
the recent Pinna f ; and in all specimens from chalk strata the cells
are empty. As the shells of Pinna, Inoceramus, and Beletnnites in
the same formations are solid, we may conclude that this difference
was original and essential.
Fig. 12. — Part of the rim of Radiolites Mortoni, Mantell, from the
Lower Chalk of Sussex. Traced from the original specimen in
the Museum of the School of Mines.
I, the outer edge ; b, the inner edge ; v, v, the dichotomous impressions.
fig. 1) it forms nearly all that usually remains of the upper valve, and all the
squamose ornaments of the lower valve. In the Hippurite it is seldom distin-
guishable ; but is quite distinct in Caprina and Caprinella. No such layer exists
in any of the Palliobranchiata. Mr. Sharpe has also pointed out that the long
prisms of the middle shell-layer are always parallel with the surface (and not
always perpendicular to the laminae of growth), like the tubes of Caprina and
Caprinella. In the Palliobranchiata the slender shell-prisms cross obliquely from
the inner to the outer surface, as shown by Dr. Carpenter. (See Davidson's Mo-
nograph of the Brachiopoda; Pal. Soc.)— Dec. 30, 1854, S. P.W.

• Specimens of Radiolites Hceninghausii which have lost their inner shell-layer
  

present internally strong transverse furrows, or lines of growth, and a prominent
ligamental ridge. The interior of the upper valve, when in this condition, shows
that the umbo of the young shell was marginal, as represented in PI. V. fig. 3. I
cannot distinguish R. acutus, D'Orb., from the young of R. Hceninghausii. —
Dec. 30, 1854, S. P. W.
t The cells of Radiolites from the English Chalk are twenty-five times as large
as those of the recent Pinna ; and those of Pinna are 250 times as large as the