﻿130 BRITISH FOSSIL CEPHALOPODA. 



appear as fine grooves of the same fineness as the riblets. The distinction between 

 grooves and riblets is so obvious that one is tempted to consider these as distinct 

 species, but all the other characters agree, and some examples may be seen in 

 which the ornaments appear in the two forms on different parts of the same 

 shell. It would thus appear that these lines were elevations both outside and 

 inside of the shell, the latter impressing the cast as strise ; somewhat as we find 

 in 0. originate. Often, also, the fossil is entirely destitute of ornament, which 

 on the above explanation is difficult to account for, unless it be the ornaments 

 die off towards the aperture and are replaced by lines of growth. Nevertheless, 

 one side of a fossil will lack the striae, and the other side exhibit them 

 perfectly. Failing, then, any other distinctive characters, these three appearances 

 must be put down to differences of preservation. The septa are more or 

 less undulating, but on the whole are direct. They are distant from -g- to -§- the 

 diameter, according to the compression, though occasionally they are wider apart. 

 Their convexity is about ^ of the same. The siphuncle is f across the diameter 

 towards the more concave side when there is any curvature. Its elements are 

 swollen between the septa, and are in earlier examples bulbous, but in the larger 

 ones butt-shaped. There are organic deposits round the neck of the septa, also on 

 their concave side, and something of the same character on the exterior of the 

 siphuncular bulbs. The abundance of small annelid borings of spiral form which 

 are constantly found on the surface of the body-chamber indicates that the animal 

 was exposed to these parasites during life, notwithstanding the apparent thinness of 

 its shell. The greatest diameter seen is about two inches, but nothing approaching 

 a complete specimen has come under observation. 



Relations. — The large size of this shell immediately separates it from the other 

 British species that are similarly ornamented, and its ornaments are much finer than 

 those of 0. jilosum. When ill-preserved, it has often been mistaken for 0. imbri- 

 catum, which is a smooth shell and of an elliptic section, and it is to this latter 

 that the isolated septal chambers, so often referred to the present species, belong. 



Distribution. — Small examples occur in the Wenlock Shale of Llyn Alwin (2), 

 Usk (2), and Dudley (1). Specimens have been seen from. the Wenlock Limestone 

 of Dudley (2), Usk (2), and Ledbury (1) ; from the Lower Ludlow of Presteign (1), 

 Mocktree (1), and Dudley (2) ; from the Aymestry Limestone of Ledbury (2) ; from 

 the Upper Ludlow of Presteign (2), Ludlow (12), Bradnor Hill (3), Woolhope (7), 

 Ledbury (5), Radnor (1), Usk (1), and Kendal (10); from, the Passage Beds, 

 Kington, and in the Upper Silurian of Kerry. 



The species is also recorded by M'Coy from Denbigh ; by Phillips, from Marloes, 

 Llandeilo, Builth, Tortworth, Abberley, and Malvern ; by Hughes, from the Coniston 

 Flags ; by Strickland, from the Upper Ludlow, Hagley ; and by Salter, from Long- 

 fields and Coalbrookdale. 



