486 Dr. H. Woodward—A Fossil in a Chalk Flint. 
I then placed the specimen before me and meditated upon it from 
time to time, a process which I have frequently found tended to 
illumination. I recalled the well-known fact that there are quite 
a number of corals and mollusca which build up body-chambers 
needlessly large for their personal accommodation, and then proceed 
to reduce them again by shutting off a portion from time to time 
by secreting a shelly partition or septum across the lower part of 
the living-chamber.' Such energy in shell-growth has been attributed 
to various causes:—(1) There may be a necessity for the animal 
to grow upwards to prevent its being immersed in sand or other 
sediment which threatened to overwhelm the sedentary mollusc or 
coral. (2) Professor H. G. Seeley suggested that more rapid shell- 
growth in Cephalopod shells was due to a periodic necessity to 
provide a larger body-chamber to accommodate the gravid oyisacs 
of the female prior to the extrusion of the ova. (3) Professor Owen 
suggested that the mantle of the mollusc, which secretes the shell, 
deposited new matter to its outer border more rapidly than it does 
to the umbonal or hinge-area. 
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Fic. 3. Section of a portion of the shell of a long-beaked oyster, Ostrea cornucopia, 
showing interior (umbonal) portion of valve filled up by a series of 
extremely thin lamellee (c), quite distinct from the compact shell-wall 
(s, s) ; @, part of the body-chamber occupied by the animal. (From 
Dr. 8. P. Woodward’s Manual of the Mollusca, 1851-6, Isted., p. 281, 
fig. 192.) 
This addition to the margin of the shell compels the animal to 
advance its body and the attachment of its shell-muscles also, in order 
to follow this forward or upward growth of the shell. To obviate 
the excess of space thus acquired the lining-mantle of the animal 
partitions off the lower portion of its body-chamber by secreting 
a series of shelly layers, very regular in Cephalopod shells, such as 
the Nautilus and Ammonite, but more or less irregular in the other 
Mollusca, such as the Water Spondylus, the Exogyra, and some 
Hippurites and Oysters. (Fig. 3.) 
1 See a series of illustrated articles ‘‘ On the Form, Growth, and Construction of 
Shells’? by the late Dr. S. P. Woodward, edited by H. Woodward, in the 
Intellectual Observer, vol.x, pp. 241-53, November, 1866 ; vol. xi, pp. 18-30, 161-72, 
1867. See also Henry Woodward, ‘‘ The Pearly Nautilus, Cuttle-fish, and their 
Allies’: Student and Inteliectual Observer, vol. iv, pp. 1-14; pt. ii, pp. 241--9, 1870. 
‘* On the Structure of the Shell of the Pearly Nautilus,’’ 1870, Brit. Assoc. Sect., 
Liverpool Meeting, p. 128. ‘*On the Structure of Camerated Shells’’?: Popular 
Science Review, vol. xi, pp. 113-20, pl. lxxxii, 1872. 
