﻿BRITISH FOSSIL CEPHALOPODA. 35 



on the previous concavity ; and throughout the whole deposit the lines are as 

 parallel as they can be to its exterior a boundary. These facts seem to suggest that, 

 when the mantle forsook the chamber to form a new septum, the shell became 

 slightly porous ; the lacunas of both layers forming in fact the pores. The air-filled 

 septal chamber would be a drying ground for water with dissolved calcareous 

 matter to evaporate in, the superfluous moisture being absorbed either by the 

 mantle through the pores of the next incomplete septum, or by the siphuncle ; some 

 of the impregnated water would also soak through the first septum and form the 

 deposit on its convexity in the previous chamber, both deposits pushing out, and 

 so becoming bounded by the thin membrane that lines the chamber ; the struc- 

 ture of which may also be impressed on the deposit, by localising the evaporation 

 to its pores. I am not thoroughly satisfied by this explanation, on account of the 

 difficulty of drying the air, which would soon become saturated, though the fact that 

 these deposits do not reach the siphuncle shows that there might be aeriform com- 

 munication between all the chambers. In any case, I regard the deposits as of the 

 nature of a disease. It may be worth noting that in the recent Nautilus the little 

 calcareous infilling of the corner between the septum and the side of the shell is 

 somewhat similar in nature and position to these, and may have a similar origin. 1 



(<?.) Truncation of the septa. — This was also first observed by Barrande, and to the 

 species which best exhibited it he gave the name Orthoceras truncatum. Of course it 

 can only occur in uncoiled shells. Amongst these are some which never show more 

 than a certain number of chambers ; the earliest preserved is evidently a late one 

 in the whole series, and on its end are formed deposits not found on the other 

 septa. A species very similar if not identical to 0. truncatum occurs in Britain 

 (see infra, p. 151) ; the septal end is continuous with the side of the shell and has 

 special deposits on it : a similar instance occurs in 0. Etheridgii. In like manner 

 the Gomphocerata never have a termination towards the earlier end, but the thickness 

 of the septal surface is double that of the others. These features are best accounted 

 for on the supposition of Barrande, that the animal had the power of breaking off 

 the end of its shell at the septa, and of depositing shelly matter on the truncated 

 end. Of course this involves the assumption that the animal could reach so far 

 outside its shell, which must have been therefore more nearly internal than in the 

 Nautilus. There is nothing improbable in this ; the coiled shells are more within 

 reach than the uncoiled, and doubtless the latter were endowed with some recom- 

 pensing powers. The numerous isolated septa which are met with in the Ludlow 

 rocks, referred to 0. imbricatum, are also most satisfactorily accounted for if they 

 were naturally thrown off one by one. 



1 Dr. Dewitz Las lately (G-iebel's ' Zeit. G-es. Naturw.' Ser. 3, Bd. iii.) described the lines which, 

 bound these deposits when they have filled the whole chamber, and therefore come in contact with 

 each other, as supernumerary septa ! But the facts adduced are easily explained as above. 



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