﻿38 BRITISH FOSSIL CEPHALOPODA. 



the horny tube which traverses the chamber and is inserted into the anterior side of 

 the next previous neck swelling, so that it is actually discontinuous at each neck. 

 It appears to be quite inelastic, as seen when dry ; but it has certainly contracted in 

 drying, as it does not fill the outer coating, and in the first chamber is drawn away 

 from the first neck, to which apparently it was not attached (PI. II., fig. 4, g). The 

 outer covering of the siphuncle is merely a crystalline exudation from the inner ; 

 several long calcareous crystals may be seen attached to the latter, and the whole 

 of the outer coat consists of similar ones held together only by their interlacing 

 (PI. II., fig. 4, A), and crossing each other at all angles. This coat is also continuous 

 from one septum to another, and enters into the concavity of each neck, even in , 

 the earliest chamber, whose neck it lines. On the convex side of each septum it is so 

 abundant that it overlaps the end of the neck and covers it on the outside, although 

 it is obviously a subsequent deposit. It is no doubt this mineral deposit which 

 yields the appearance of continuous siphuncles in many fossil shells, in which 

 a more careful examination shows the terminating neck of the septa to be com- 

 paratively short. Unfortunately, this simple structure yields very little assistance 

 in determining the nature of the complex bulbous siphuncles so often met with in 

 Palssozoic Orthocerata. Fossils containing these have been long ago described under 

 the generic titles Actinoceras and Ormoceras. In the former of these the siphuncles 

 have been stated to consist of two tubes, of which the inner was connected with 

 the outer by radiating plates. 1 Such plates Barrande has well shown to have no 

 existence. The appearance is brought about by the growth of organic deposits 

 round the necks of the septa, and their increase till they actually meet, and each 

 forms half of the bulb. In this process it is supposed that the siphuncular tube, 

 which was originally of large size, has its diameter greatly diminished and thrown 

 into folds ; while the superabundant membrane is squeezed flat in the middle 

 between the two halves of the bulb, and is thus thrown into radiating folds, whose 

 contents have been mistaken for actual plates. That this explanation is in the right 

 direction, at least, a careful examination of any so-called Actinoceras, or of Barrande's 

 plates, must convince any one who will take that trouble ; and from this it follows 

 that Ormoceras is the same thing when the process has not continued so long, and 

 is therefore incomplete. Barrande, however, considers this process to take place 

 entirely on the outside of the siphuncular tube, against which idea are several 

 opposing facts. The outside of these bulbs has quite as definite an outline as the 

 inner tube, or even more so ; the concentric deposits round the necks may be easily 

 seen undisturbed, till they grow into contact with the next, or till they have been 

 stopped on the outside by this definite line ; but, more particularly, some Orthocerata, 

 as 0. giganteum, show plainly a distinct arrangement of folds on the outside, which 

 pass inwards in the middle of the bulb ; and hence the same membrane which makes 

 1 See Stokes, Trans. Geol. Soc, Series II., vol. v. p. 705. 



