File:The Quarterly journal of the Geological Society of London (12711328015).jpg

46
PROCEEDINGS OF THE GEOLOGICAL SOCIETY.
Genera related to Hippurites.
In searching out the affinities of a problematic fossil shell, it is
desirable to inquire, first, whether any similar, but less abnormal,
forms occur in the same stratum with it, or in formations imme-
diately older or newer. For it may be doubted whether any quite
isolated types exist in nature ; and, although no well-disciplined
naturalist dreams of the transmutation of organic forms, yet by those
who regard genera as "ideas of the creating mind," there is held to
be a relation of interdependence between those "ideas," as regards
their development both in time and space, giving rise to a succession
of forms, which may easily mislead a superficial observer to suppose
they are related in the way of ancestry or descent.
Therefore, although we disbelieve the doctrine of transmutation,
our first inquiry is — Are there any fossil shells which look like the
progenitors or descendants of the Hippurite ? We think it may be
shown, that, by a complete series of cognate forms, the Cretaceous
Hippurites are connected with the Oolitic Dicerata and the Tertiary
ChamcB.
These forms belong to at least five genera ; and some of the species
are more problematic and extraordinary than the Hippurite itself.
Radiolites. — The genus most nearly related is the Radiolites of
Lamarck .SphcBvulites of De la Metherie) .
Figs. 10 & 11. — Radiolites mammillaris, Matheron. Reduced \.
From the Lower Chalk, S. Mamest, Bordogne.
Fig. 10.
Fig. 11.
Fig. 10. Interior of lower valve. Fig. 11. Interior of upper valve. (See also
figs. 13 & 14.) — I, ligamental inflection ; m, pallial line ; c, c, cartilage-
pits ; a, a', adductor impressions and processes ; t, t, teeth and dental
sockets ; u, umbonal cavity.
In general form the Radiolites resemble the Hippurites, but are
more squamose or foliaceous externally ; and the upper valve is not
porous, differing but little in structure from the lower valve*. The

• Mr. D. Sharpe has called ray attention to the existence of a third, superficial,
  

shell-layer in all the genera of Rudisfa, similar to the " sub-epidermal " layer,