472 Mr. VALENTINE on the Development of the Theca, 
although it follows the law above stated, is very anomalous in other respects. 
It is a horizontal membranous ring, formed between the opercular membrane 
and the horizontal portion of the columellar membrane. This situation pre- 
cludes the possibility of its having been formed by the separation of the 
internal layer of the operculum. 
It is now necessary to describe the development of the sporules. The 
period at which this process commences is rather uncertain ; most probably it 
begins at the time of the separation of the columellar membrane from the 
columella. Dr. Hooker in the Flora Londinensis, vol. iv. fasciculus i., under 
* Diphyscium foliosum,” has this passage: “It would be curious to ascertain, 
were it possible, what becomes of the substance forming the cellules in the 
early state; for the ripe seeds are quite free and unconnected, yet not sepa- 
rated by any membranous substance such as the walls of the cellules appear 
to have been formed of. On the contrary, they occupy a cavity around the 
columella, which appears evidently to be nothing more than the remains of 
the cellular and pulpy substances in which the seeds have not been per- 
fected, and which, as we may consequently expect, when dry, shrinks up into 
an angular axis or columella, as it is called by Hedwig and other muscolo- 
gists.” Mr. Brown, in the Linnean Transactions, vol. x. p. 315, says, in 
speaking of what he names tbe placentation of the seeds: “ That in some 
cases the seeds may be formed in a much greater portion of the columella 
than in others: and it is even not improbable that in certain cases its whole 
substance may be converted into seeds: or, to speak more accurately, that it 
may produce seeds even to the centre, and that the cells in which they were 
probably formed may be reabsorbed.” From these passages it appears that 
their authors consider the seeds or sporules to be formed in the columella, 
and even of its very substance. Dr. Greville and Mr. Arnott, in their Memoir, 
object to the opinion that the columella, in the ripe theca, is merely a con- 
traction of the debris of the sporular mass, from the regularity of figure which 
it often retains, and also from its being sometimes tubular; a fact which, they 
say, is irreconcileable with the notion of contraction. My observations have 
convinced me that the sporules are formed from a gummy fluid, which is 
secreted either by the columella or columellar membrane (most probably by 
both), and that this secretion becomes cellular by the gradual separation of 
