File:The Quarterly journal of the Geological Society of London (12711785514).jpg

48
PROCEEDINGS OF THE GEOLOGICAL SOCIETY.
The rim of this species and some others (but not of all the Radio-
lites) is marked vAth. bifurcating impressions, which radiate from the
inner to the outer margin (fig. 12). They owe their definition to a
change in the form of the cells.
The inner layer of shell is often wanting, being only indicated by a
space between the outer wall and the calcareous mould of the original
interior. These moulds (figs. 15, 16), called Birost rites by Defrance,
have greatly puzzled naturalists, especially when imperfect *.
In the British Museum there are several specimens of i?. calceo-
loides and R. mamrnillaris in which the inner layer of shell is re-
placed by spar, whilst the interior was filled with soft chalk, and
allowing the separation and development of the valves.
The interior of the lower valve of R. mamrnillaris (fig. 10) exhibits
no inflections of the outer wall, or only a slight ligamental ridge ;
the cartilage-pit is deep and furrowed, and divided by an inflection
of the inner wall. The dental pits are deep, subequal, and strongly
grooved. The muscular impressions are shallow, striated, and nearly
equal.
The interior of the upper valve (figs. 11, 13, & 14) has an um-
bonal cavity turned towards the hinge, and slightly mider-cutting it.
In young specimens it is deep and conical, but becomes shallow,
or completely filled up, with age. The teeth are straight and pro-
minent, fitting accurately the grooves in the sockets.
Figs. 13 & 14. — Two side-views of the upper valve of Radiolites
mammillaris, from the same specimen as Fig. 1 1 .
Fig. 13.
Fig. 14.
I
I, ligamental inflection ; t, t, teeth ; a, a', muscular processes.
Each tooth supports a curved apophysis corresponding in form
to the muscular impressions in the lower valve. In aged specimens
the apophyses nearly rest upon the impressions ; they are, however,
seldom so prominent as in this species f ^^ — '^ ^'" ""'" ^ '""' """
There is no
longer
any

• G. Sowerby figured a Birostrites incequilobus in his * Genera of SheDs,' and
  

rightly regarded it as the mould of a shell related to Diceras.
t The specimen of R. calceoloides in the British Museum, like that described by
M. Deshayes, Bull. Soc. Geol. France, 2 ser. viii. 12 7, has lost almost all character