﻿BRITISH FOSSIL CEPHALOPODA. 159 



very small. The surface is not preserved. The septa are direct and have a con- 

 vexity of about ^ the diameter. Their distance is about f the long diameter. The 

 siphuncle has a diameter of y the same line and lies f across the long diameter, 

 which is li inches. From the Lower Ludlow of Aymestry. In the Museum of 

 the Geological Society. 



General Description. — Examples referred to this species, on account of the 

 remoteness of their septa, show sufficient variation in that element and in the 

 apparent rate of increase of the shell, to render it possible that better specimens 

 would prove the existence of more than one species. The section is elliptic, and the 

 difference of diameters becomes more marked by compression. The rate of increase 

 in flattened or compressed examples is 1 in 4 and 1 in 8 respectively ; and in a small 

 one, 1 in 7. The body-chamber is 3^ times its flattened basal diameter; it remains 

 uniform to the aperture, which is simple, but imperfectly preserved. In the young 

 shell there were some transverse strige, not observed in larger specimens. The 

 septa are direct, and have a convexity of less than -| the diameter. Their distance 

 is always greater than ^ the same, but in none are they seen quite so remote as in 

 the type. The siphuncle is excentric on the long diameter about f across, but also 

 in one example yq across the short one, — an irregularity probably due to compres- 

 sion. Its structure was either more or less bulbous, or there was a deposit around 

 the neck of the septa. Its size is from ^ to T the diameter. The greatest diameter 

 is If- inches, and the greatest length seen is 8 inches. 



Relations. — Now that this species is better known, it cannot be thought to have 

 any close relations to 0. ludense, as supposed by M'Coy. In the specimen referred to 

 by Salter, as showing longitudinal lines, the supposed septal lines are cracks. Several 

 of Barrande's species may differ from this in details on which our material affords 

 no information, but I think 0. migrans will in any case be among its synonyms. 

 The Orthoceras Thomsoni of Barrande, figured as having come from the Silurian of 

 Scotland, but from an unknown part of it, is too imperfect a specimen, being worn 

 away on the outside, to make any certain determination of; but in spite of its 

 apparently more central siphuncle, it very probably represents the present species. 

 This differs from 0. vagans in its more regularly spaced septa and less central 

 siphuncle. 



Distribution. — In the Wenlock Shale, Rebecca Hill (1) ; in the Wenlock Lime- 

 stone, Malvern (1) ; in the Lower Ludlow of Ledbury (1) and of Aymestry (3) ; 

 and in the Upper Ludlow of Aymestry (1). 



It is also recorded by Grarner from the Lower Ludlow of Hay Head. 



