Dr. H. Woodward—A Fossil in a Chalk Flint. 487 
In some corals the lower portion of the corallite chamber may be 
shut off by nearly regular simple horizontal septa, as in Favosites, 
or the interior may be filled by irregularly formed meniscus-like 
cells filling up the central cavity below each polype, as seen in the 
Zoantharia Rugosa. In Ostrea cornucopia (Fig. 3), and in a species 
of oyster from the Tertiary of Cerigo, Jonian Isles, in Mr. W. J. 
Hamilton’s Collection (Brit. Mus. 34033), a large portion of the interior 
of the shell is shut off by numerous irregular septa forming, as in 
Spondylus, water-cavities within the shell. These inner septal shell- 
layers are quite distinct from the outer shell-wall, they are much less 
compact, the lamelle are extremely thin, and they are often dissolved 
out entirely in the fossil forms. 
In those singular molluscs the Rudistes we have forms like 
Hippurites organisans, Montf., in which a portion of the lower 
deep valve is partitioned off, like an Orthoceras, by a succession 
of almost regular septa, the intervening spaces forming water- 
chambers (Zittel, Paleont., p. 282, fig. 632, 1895). 
Gan oe 
Fic. 4. Part of the internal mouw/d taken from the interior of a shell of Radiolites 
Mortoni, Mantell (reduced), representing some of the water-chambers 
of the original shell, perforated by cliona. +, 7, joints produced by the 
decomposition of the septa; a, furrows produced by adductor ridges ; 
t, furrows produced by the dental ridges. From 8. P. Woodward’s 
article in Q.J.G.S., vol. xi, pl. v, fig. 2, 1855. 
Fic. 5. Diagrammatic vertical section through shell of Radiolites, showing s, s, the 
outer shell-wall composed of prismatic cellular structure, the prisms being 
vertical to the shell-laminze and minutely subdivided. ¢, section of the 
inner layer composed of transverse lamelle, which are extremely thin, 
and are separated by intervals, like the water-chambers of Spondylus, and 
the similar spaces in the umbonal cavity of the long-beaked oysters. 
(See Fig. 3 supra; compare also with Fig. 1.) 
In the British Cretaceous species Spherulites (Radiolites) Mortons 
the outer shell-wall is very thick and consists of dense prismatic 
cellular structure, like the recent Pinna. The lower central part 
of the valve has been shut off from time to time by a series of 
somewhat irregular meniscus-like septa, deposited by the mantle and 
built wp in succession one upon another. In a specimen described by 
