﻿126 BRITISH FOSSIL CEPHALOPODA. 



were thicker towards one side than on the other, and where they were thicker there 

 was also a plate which connected the siphuncle with the side of the shell throughout 

 the whole chamber. The greatest diameter is ^ inch, and the length is ^ inch. 

 From the Tilestones of Horeb Chapel. In the Museum of the Geological Society. 



General Description. — The section in some examples appears to be elliptical, with 

 the axes in the ratio of 3 to 2 ; but as the shorter diameter corresponds to the 

 exposed side, this is probably due to weathering. The rate of increase is uniformly 

 1 in 6. Nothing but septate fragments have been seen. As to the surface orna- 

 ments, Phillips's type specimens of 0. textile, while they show the septal characters 

 of the present species, have not been observed by me to show any ornaments : but 

 doubtless the fragment figured by Phillips belongs to the same as the rest, in which 

 case the surface must have been marked by lines crossing each other so as to form a 

 network (fig. 12). The septa are direct and their convexity slight. In very minute 

 shells they are distant as much as \ the diameter (fig. 11), but they must rapidly 

 grow closer, as all ordinary examples have them from \ to ^ of that line apart. The 

 siphuncle is near the centre, bat lies rather nearer the side which has the plate (fig. 10). 

 It is stated to be lateral by Phillips in his 0. textile, though one figure gives it correctly 

 near the centre. The feature which induced the idea of its being lateral is as 

 follows : — On one side of each septal chamber is a longitudinal elevation in the cast, 

 which becomes more prominent towards the base, and in fact forms one of the 

 ordinary discontinuous normal lines, and is unconnected with the siphuncle, though 

 often looking like one. Towards this side the septa thicken and leave in the cast a 

 wider gap between the hollow parts of the chambers (fig. 9), which are now filled with 

 matrix. From the same side starts a plate, or probably two, which run together to the 

 siphuncle, though diverging towards the surface, just as though the shell-secreting 

 mantle instead of being pierced by the siphuncle had been forced to go round it, 

 and at the same time to stay as close as possible to the circumference. The plate is 

 thus deposited towards the base only of each chamber, by the apposed sides of the 

 mantle ; for though the cast of the concave part of the chamber shows the marks of 

 this deposit, that of the convex part shows none. The phenomenon is therefore one 

 confined to each chamber, and is not the same as the backward prolongation of 

 the body-chamber in Tretoceras. It may, however, be due to the organic deposit. 



Relations. — There cannot be the slightest doubt that the species described by 

 Phillips as 0. textile is the same as Sowerby's originally described cast ; they differ in 

 no single respect, though mutually illustrative. It may be mentioned that the 

 supposed lateral siphuncle of 0. Steinhaueri, from the Carboniferous rocks, is really 

 the same surface depression or discontinuous normal line, and similar appearances 

 are amply illustrated by Barrande. If the specimens seen are adult, this must be a 

 cancellated species, but it may be only cancellated in youth, and the adult form be 

 less peculiarly marked, which would account for the ornaments being so seldom 



