﻿9 8 BEITISH FOSSIL CEPHALOPODA. 



and the rate of increase is 1 in 12. There is no curvature whatever. The 

 ornaments are acute ribs, though rather rounded in the cast; they are scarcely 

 separate, are quite direct, and distant ^ to f the diameter apart. When well 

 preserved, there are sharp riblets parallel to the ribs, somewhat imbricating 

 downwards. The septa are parallel to the ribs, and lie in the interspaces; their 

 convexity is very small, indeed they are remarkably flat. The siphuncle is small, 

 and nearly, if not quite, central. The greatest diameter seen is i inch, and the 

 length 2-f inches. 



An example which agrees with this in all respects, except in having a greater 

 convexity of septum than usual, shows the characters of the body-chamber. It is 

 twice the length of the basal diameter, and contracts notably towards the aperture. 

 The last four ribs are also crowded into a much smaller space. 



Relations. — It may seem doubtful whether two species so similar to each other 

 as 0. ibex and 0. tracheale can be retained : still it must be admitted that there 

 are great differences between such forms as fig. 5 and fig. 7, though figs. 3 and 4 

 may be somewhat intermediate. The chief difference is in the directness of the 

 ribs in 0. tracheale, which was apparently made the basis of Sowerby's original 

 separation. We cannot rely on the acuteness of the ribs, unless we are prepared 

 to multiply species, for either form of rib appears possible in each without intro- 

 ducing other distinctions. Both these species are essentially straight, and have no 

 relation to the curved forms with which they have been confounded, as may be seen 

 at once by an inspection of fig. 10 compared with figs. 5 and 7. Orthoceras per- 

 elegans is a name for which there is no room after the proof of the transverse 

 ornaments of both this and 0. ibex. 



Distribution. — In the Lower Ludlow of Mocktree (1) and Aymestry (3) ; in the 

 Upper Ludlow of Llandovery (1), Ludlow (7), Richard's Castle (1), and Usk (4) ; 

 in the Tilestones of Trecastle (2), and from the Upper Silurian of the Pentlands. 



M'Coy also records it from Ludlow Beds at Kendal, Llangollen, and from the 

 Wenlock Shale at Sedbergh. Phillips records it from Llandeilo ; and Lap worth 

 and Wilson from the Riccarton Beds. There are specimens from the Lower Ludlow 

 of Ledbury (5) and Dudley (1), which seem to differ from this only by the septa 

 corresponding to the ribs, but they may really belong to 0. Grayi. 



Orthoceras tenuiannulattjm, M'Coy, PI. V. figs. 9, 9a. 



1851. Cycloceras tenuiannulattjm, M'Coy, 'Ann. Nat. Hist.' Ser. ii. vol. vii. p. 45. 



1852. „ „ M'Coy, 'Palseozoic Fossils,' pi. 11, fig. 31. 

 1873. Orthoceras texuiannulatum, Salter, ' Camb. ai.d Silur. Fossils,' p. 173. 



Syn. 1852. Cycloceras ibex, M'Coy, 'Pal. Foss.' p. 319 (part). 



— Orthoceras ibex, Salter, 'Pal. Fo:-s.' Appendix A, p. vii. (Not of Sowerby.) 

 1854. „ vertebrale, Morris, ' Cat. Brit, Foss.' p. 312. (Not of Hall.) 



