﻿BEITISH FOSSIL CEPHALOPODA. 99 



1873. Orthoceras ibex, Salter, ' Canib. and Silur. Foss,' p. 186. 

 Query 1865. Orthoceras tenuiannulatum, Haswell, ' Silur. Foss. of Pentland Hills,' pi. 1, fig. 3, 

 p. 25. 



Type. — The section is now flattened, so that the radii are as 1 to 3. Its rate 

 of increase in the longer diameter is 1 in 16. No signs are here given of the body- 

 chamber, the form of the aperture, the septa, or the siphuncle. The ornaments are : — 

 first, very clearly separated rounded ribs, the interspaces being wider than they, and 

 but slightly concave ; these are nearly direct, having a slight concavity towards 

 the aperture on the broad side, and are distant \ the flattened diameter : secondly, 

 there are fine longitudinal raised lines, 20 per line, and at the smaller end the tops 

 of the ribs have two or three transverse lines. The large ribs die off for 

 about \ inch from the larger end, leaving only a striated surface. The greatest 

 diameter (compressed) is \ an inch ; the length seen is 2 inches. From the Lower 

 Ludlow of Aymestry. In the Woodwardian Museum. 



General Description. — It is probable that the true section was slightly elliptic, as 

 one example, apparently uncompressed, has that form ; they are mostly, however, 

 found with very unequal diameters. There is not the slightest curvature in any 

 example ; and if some small ones are rightly included, we see almost to the 

 commencement. The rate of increase, even in the most compressed, is never 

 greater than 1 in 9, more usually 1 in 14, but varying down to zero. No 

 characters of the body-chamber or of the aperture have been seen. The ornaments 

 are always the characteristic ribs of the type, occasionally a little more acute than 

 usual ; they are generally direct, but a maximum obliquity of 5° may be present. 

 On larger specimens they are distant \ to \ the diameter; but at the smaller 

 end they are closer, and may, by compression, be only \, or even a less fraction of 

 the diameter apart. The longitudinal riblets vary chiefly with the diameter ; 

 generally they are 12 to 14 per line, but, as in the type, they may be more 

 numerous, or at a large diameter reduce to 5 per line. No transverse lines 

 are seen in most, but the smaller ones have some on the ribs, and a few small 

 examples agreeing with this in every other respect, which may be assumed (for 

 certain proof is wanting) to be the young, have an equal close cancellation, the 

 transverse lines not being quite parallel to the ribs. Towards the aperture the 

 larger ornaments give way to irregular ridges of growth. The septa are seen 

 in two examples to lie in each interval between the ribs, but no further details 

 are observable. The greatest diameter is 13 lines, and the greatest length seen is 

 2| inches. An example of this species shows, by the arrangement of its surface 

 ornaments, that the shell was broken and repaired during life. 



Relations. — This species is well characterised by the longitudinal finer orna- 

 mentation and the separateness of its ribs. Salter, in the second volume of the 

 ' Memoirs of the Geological Survey,' first stated that the 0. ibex of Sowerby had 



o 2 



