﻿162 BEITISH FOSSIL CEPHALOPODA. 



the shell; it is never seen lateral, and would seem to be really sub-central. The 

 greatest diameter of the bulbs is from J- to -f- the whole diameter. These bulbs, 

 which constantly occur alone, and the characters of which are therefore important, 

 are flattened spheroids lying between the septa. The transverse diameter is usually 

 twice the longitudinal, and the aperture between them is equal to the latter ; but 

 when they are flattened, they become narrower and communicate by wider openings. 

 That there is a double membrane seems proved by different specimens : one shows on 

 the outside of the bulbs about 30 longitudinal bands (fig. 7), and another shows that 

 within the bulbs there was a membrane drawn up in folds to the centre and com- 

 municating from the front to the back of the bulbs (fig. 8). The greatest length 

 preserved is 3^ inches, and the greatest diameter 1\ inches. 



Relations. — By universal consent the 0. crassiventris of Wahlenberg is the same 

 as Schlottheim's 0. cochleatum, while, as stated above, 0. nummularium may be 

 merely a fragment representing the last two chambers only. More than one species 

 may possibly be included here, but they all differ from 0. conicum in the slower 

 rate of increase, and from Tretoceras bisiphonatum by their flatter siphuncular beads. 



Distribution. — In Lower Silurian rocks, Gwenfydd (2) ; in the Lower Llandovery, 

 Mullock (1) ; in the Upper Llandovery of Llandovery (1), Bogmine (1), Marloes(l), 

 Tortworth (1), Eastnor (3), Norbury (1), Charfield (1), and Cerrig-y-druidion (1); 

 in the Wenlock Beds of Tortworth (1) ; in the Upper Ludlow of Kirby Lonsdale (2) ; 

 and the Tilestones of Horeb Chapel (3). 



Subgenus Endoceras. 

 Orthooeras (Endoceras) Brongniartii, Troost, PI. XVII. figs. 1, la. 



1837. Coxotubularia Brongniartii, Troost, ' Mem. Soc. Geol. de France,' vol. iii. pi. 9, fig. 2, p. 89. 

 1843. Orthoceras Brongniartii, Portlock, 'Geol. Bep.' pi. 28, fig. 4, p. 368. 

 1857. „ „ Salter in Murchison's ' Siluria,' p. 199. 



Type. — The figure given by Troost represents a large shell increasing at the 

 rate of 2 in 13. The septa are undulating, and distant about ^ the diameter. He 

 states that the section is elliptical and the siphuncle lateral. It does not appear, 

 however, that it is so lateral as to have part of its boundary coinciding with that of 

 the shell ; its diameter is almost | of the whole, and it is surrounded, as indicated by 

 the generic name, by the necks of the septa. The length is 6| inches, and the 

 greatest diameter is 3| inches. From the Lower Silurian of Tennessee. 



General Description. — The specimen figured by Portlock, and others examined, 

 agree very closely with this description. The section is elliptic, the axes being as 

 9 to 7. The rate of increase of the longer diameter is 1 in 7 to 1 in 5. The body- 



