﻿BEITISH FOSSIL CEPHALOPODA. 39 



the radiating plates is continued over the surface of the bulb. These deposits must 

 therefore take place between the membranous siphuncle and its brown horny- 

 envelope ; as the deposit grows, it detaches more and more of the envelope in the 

 region of the septum, till at last the two are only connected in the centre of the 

 interval. In some cases, as when the bulbs are not subdivided in the centre, the 

 two must have been altogether detached, and the deposit became continuous between 

 them. In other cases they must have remained attached along longitudinal lines, as 

 when there are longitudinal plates within or folds on the outside of the bulb. These 

 deposits are much too solid to be compared directly to the crystalline external coat of 

 the siphuncle of Nautilus pompilius, but they are exudations of the same general nature. 

 Barrande draws a line in his classification between those species whose siphuncles are 

 nummuloid, or with the transverse axis greater than the longitudinal, and the cylin- 

 drical, which are narrow. It is, however, quite an arbitrary line, and the essence 

 of his explanation of the origin of the bulbs is that there is no real difference 

 between the two kinds. In some American and Canadian forms siphuncles have 

 been figured, and actually exist, in which the earlier part is partitioned off by conical 

 septa in the same way as the shell itself is ; but none occur in British rocks, unless 

 the doubtful Piloceras be of this nature. 



(d.) Its functions. — The old idea of its expanding and contracting, so as to 

 increase or diminish the density of the gas in the chambers, has long ago been 

 exploded, on account of the crystalline friable coat, the continuity of the necks of 

 some species, and the solidity of the organ in others. 1 It is not a ligament or a 

 muscle, for it is hollow and contains an artery. The only plausible functions at 

 present assigned to it are those of nourishing and keeping alive the septal portion 

 of the shell, or of secreting fresh supplies of the gas which may escape. The first 

 of these is due to Searles Wood, 2 and is adopted by Professor Owen, 3 and the second 

 to Keferstein. 4 Barrande, while giving no theory of his own, objects to these on 

 several grounds. The first is the existence of a double envelope, which would no 

 doubt prevent the ramification of veins, but not the exudation of moisture or gas ; 

 another is that the septal necks form a continuous tube ; but this is scarcely the case, 

 for the friable, and hence porous, outer layer always intervenes between one neck 

 and the next septum. Hence, I think, both of the above functions may be performed, 

 not by the ramification of arteries on the septal surface, of which there is no sign in 

 the bounding pellicles, but by the continual porosity of the exposed parts, by which 

 those pellicles are kept moist, and the organic deposit, both on septa and in the 

 siphuncle, rendered possible. 



1 Yet it has been given as the true explanation without discussion by Dr. Wright, '-Lias 

 Ammonites ;" Pal. Soc. 1880. 



2 Edwards' "Eocene Mollusca ;" Pal. Soc. 1849. 



3 Proc. Zool. Soc. 1879. * < Palseontographica,' 1871. 



