﻿82 BRITISH FOSSIL CEPHALOPODA. 



certainly circular, though generally compressed. The rate of increase is usually 

 1 in 10. No body-chamber is seen. The ornaments slightly undulate, and are 

 distant ^ the diameter. They do not seem to be less, but perhaps even more acute 

 when young ; but the beekised specimens are small ones. The septa have but 

 slight convexity and are from £ to -^ the diameter apart, appearing always to 

 correspond to ribs. The siphuncle is seen to be central, at least when the shell is 

 small, and then has a diameter of -^ to f of the whole as seen on the end. This 

 consists of the renal obstructions which become amalgamated with the septa, and 

 are almost lost, and of a comparatively large- sized inner tube, untouched by the 

 septa and slightly folded longitudinally ; these features are seen in all specimens 

 showing the interior, and prove that we have here a very remarkable type with a 

 siphuncle of two distinctly preserved layers, the outer one being separated by 

 obstructing deposits from the inner one. The type is the largest of the specimens, 

 and there are some from £ inch downwards. 



Relations. — Salter compares this with 0. multicameratum of Hall, but I regard 

 the present as a ribbed species ; and in any case the siphuncular arrangement, if 

 rightly interpreted, separates this from Hall's species, and indeed from all 

 comparable ones. 



Distribution. — From the " Lower Llandeilo" Limestone of Durness (8). It is the 

 commonest Cephalopod of these rocks, and I have seen several others besides those 

 critically examined. 



Orthoceras baculoides, Blake, PI. III. fig. 2. 



Type. — The section is nearly circular, and the rate of increase is 1 in 18. Only 

 a fragment is seen, showing the surface. This has semi-annulations sloping back 20°, 

 and dying away on the lower side ; these are ^ the diameter apart, and there are 

 feeble intermediate lines. The siphuncle is central and large. The length is 

 1\ inches, and the greatest diameter 5 lines. From the " Llandeilo" Limestone 

 of Durness. In the Museum of Practical Geology. 



General Description and Relations. — Two other examples, slightly compressed into 

 an oval, show an equally slow tapering, and one shows a central siphuncle ; these 

 features seem to separate the specimens from 0. durinum, whose exterior surface is 

 not known, while in 0. arcuoliratum the ornaments are stronger. Nevertheless, 

 this is an imperfectly known form. 



Distribution.— Only found in the Durness Limestone (3). 



