﻿36 BRITISH FOSSIL CEPHALOPODA. 



The utter absence also in all the known specimens of Ascoceras of the remains of 

 any of the undistorted chambers, can scarcely be due to accidents of preservation ; 

 but these must have been originally easily separable. The shell of such forms as show 

 truncation is seldom preserved, and it was perhaps thinner than that of others. 



8. The Siphuncle. 



The extreme importance of this organ in the eyes of some naturalists may 

 be judged of by the fact that Barrande occupies more than 1100 large quarto pages 

 in the discussion of it. We have to consider — 



(a.) Its position. — One of the chief characteristics of the Nautiloidea is that this 

 is variable, whereas in the Ammonitoidea it takes up a constant position on the 

 convex side of the shell. This is correlated with its variability in other respects. 

 In the recent Nautilus, commencing as it does at the base of the nucleus opposite 

 the end of the cicatrix, it maintains a pretty central position throughout, passing in 

 a straight line through the earlier chambers so as to form a polygon, but taking the 

 curve of the shell in the later ones. 



Although there are families entirely or chiefly composed of species with non- 

 central siphuncles, still that position is the preponderant one, and Barrande shows 

 by an enumeration of all known forms that out of 1500 there are 500 with central 

 and 418 with subcentral siphuncles. Moreover, except the Clymenidce, there is no 

 family of Nautiloids, or even genus with more than one species, which can be charac- 

 terised by the position of its siphuncle. Nevertheless, in such genera as Orthoceras 

 and Nautilus it is more usually central ; while the Cyrtoceras, Phragmoceras, and Tro- 

 choceras have it more commonly marginal. If we seek for the connection between 

 the position of the siphuncle and the form and character of the shell, no constant 

 relation can be found, though certain tendencies may be observed which indicate 

 some relation between them, and hence the mere observation of the proportions of 

 central and eccentric positions on the long and short diameters will be of little use. 

 In the first place, we may consider the siphuncle as normally always to be 

 situated in the plane of symmetry. Hence when a shell, coiled or uncoiled, 

 has the long axis in that plane, the siphuncle must be on the long axis, and it 

 is surely the form of the shell which governs the siphuncle, and not the siphuncle 

 the shell. The Phragmocerata, for example, are always compressed shells : hence 

 their siphuncle is always in the long diameter. The position is also related to the 

 obliquity of the septa, or of the ornaments, or of both. When the septa are 

 oblique, we may generally anticipate an eccentric siphuncle in the line of slope, 

 and the more eccentric as the slope is greater ; sometimes the siphuncle is on the 

 semi-diameter nearest the aperture, but more usually, especially when the eccen- 

 tricity is not great, it is on the more remote one. Exactly the same may be 

 said of the ornaments, but their slope is more constantly backwards towards the 



