﻿34 BEITISH FOSSIL CEPHALOPODA. 



usual crystalline one, which has been shown by Barrande to be due to the animal 

 and not to fossilisation ; and he has been followed in this by other naturalists. All 

 one's prejudices are against the organic nature of these deposits ; but the evidence 

 leaves the probability strongly in favour of it. (See PI. XII.) They lie on both sides 

 of the septa, the one nearest the aperture being usually the smallest, since it does 

 not extend so far from the side. The outlines are irregularly mammillated and parallel 

 to the septa, ending off in a prominent curve at last, and sometimes having additional 

 outlying masses. Their structure has a concentric appearance, like chalcedony ; and 

 they are generally of a brown colour, consist entirely of carbonate of lime, and are 

 most abundant towards the apex, and on one side of the shell. These characters are 

 so constant as in themselves to point to a connection with the animal, and they are 

 the proofs offered by Barrande. To them may be added the following. Something 

 similar to this must have existed in 0. conicum and others from the Llandovery sand- 

 stones, in which there is a hollow at the same place ; and since the effect of fossilisa- 

 tion has been to consume all the calcareous matter, and this with the rest, it can 

 scarcely have been also to produce the deposit. In an example of the carboniferous 

 Actinoceras giganteum, a shell which often shows this feature, a horizontal section shows 

 the structure to be peculiar ; it is divided into polygons by clear narrow spaces, and 

 the centres filled with excessively fine irregularly radiating dark lines, like so many 

 tassels, or tufts of Batrachospermum. This may be a mineral structure, like the moss 

 agates, but it is certainly not an ordinary result of fossilisation. The deposits are 

 bounded by a non-calcareous thin band, with patches of black here and there. The 

 mass of the great siphuncle, for which this species is noted, is composed of material 

 of similar appearance and colour ; and the boundary is the same thin band with 

 dark particles. This similarity is a great argument in favour of these deposits 

 having a similar origin. But once more : in the process of fossilisation of the specimen 

 examined, the thin septa, where unprotected by them, are usually broken down ; 

 but sometimes the deposits are also broken and their fragments carried with those 

 of the septa : they existed therefore before the invasion of the crystalline matter into 

 the interior. For these reasons I am compelled to regard them as belonging to the 

 animal and formed during life. But how formed ? Barrande regards them as 

 mantle secretions of limited extent, and founds on their continuity along the inside 

 of the shell an argument against the rapid motion of the animal forward ! Their 

 perfect distinctness from the septa, which are marked off by clear lines, their utter 

 irregularity and inconstancy in the same species and the same specimen, and their 

 difference in structure, seem to me to entirely preclude this idea. It is, however, 

 difficult to give a satisfactory account of them. The deposit on the concave side of 

 the septum is continued in most cases along the inside of the shell, until it is cut off 

 by the next septum, whence the latter must have been deposited first ; the deposit on 

 the convex side usually gives similar indications of having been deposited after that 





