﻿BRITISH FOSSIL CEPHALOPODA. 117 



Relations. — This species has been by many, though not by all, considered to be a 

 variety of 0. annulatum, and it is undoubtedly closely allied to it. It seems to me, 

 however, to be worthy of a distinct name, for the following reasons : — The least 

 ribbed varieties of 0. annulatum always give some indication of the ribs, how- 

 ever feeble, but in those referred to 0. fimbriatum no sign of such transverse 

 risings of any kind is seen over any part of the shell, even when of small size. In 

 the former also the longitudinal elevations are quite exceptional, in the latter they are 

 the rule. Sowerby states as one of the differences that the festoons in 0. annulatum 

 end on the convexities, and in 0. fimbriatum on the concavities. This is not 

 universally the case ; they lie irregularly ; but in the latter species the festoons are 

 much more subordinate to the longitudinal ornaments. In septal and siphuncular 

 characters they are very much alike, but in the present species the elements of the 

 siphuncle are shorter, and, if we are right in referring 0. Brightii to it, the deposits 

 at the junction with the septa are a phenomenon not found in 0. annulatum. 



Distribution. — In the Wenlock Limestone of Dudley (4), Malvern (1 and 9 

 siphuncles), Ledbury (1), Usk (1), and Eastnor (1 siphuncle) ; in the Lower Ludlow 

 of Ledbury (5), Aston (1), and Ludlow (1 siphuncle) ; and in the Aymestry 

 Limestone of Woolhope (1 siphuncle). 



Section Lineati. 



Orthoceras argus, Barrande, PI. VI. figs. 13, 14. 

 1868. Orthoceras argus, Barrande, ' Syst. Sil. de Boheme,' pi. 325, p. 476. 



Type. — The section is generally circular, but is occasionally somewhat elliptic. 

 The rate of increase is 1 in 7. The body-chamber has a length equal to 2^ times 

 the basal diameter. The aperture is direct and simple, but below it is a slight 

 depression of the shell. The ornaments are sharp, direct riblets, about 40 per line. 

 The septa are direct, and distant f- the diameter ; their convexity is less than half 

 the same line. The siphuncle is always near, but sometimes not quite at, the 

 centre ; its elements are cylindrical, and its size moderate. The greatest length is 5 

 inches, and the greatest diameter § inch. From the stage F of the Upper Silurian. 



General Description. — The British examples referred to this species agree, so far 

 as they go, very fairly with the above description. The section is nearly circular, 

 the ratio not being greater than 16 to 15. The rate of increase is between 1 in 7 

 and 1 in 8. The ornaments are direct sharp riblets of considerable regularity, 

 and from 32 to 36 per line, perfectly observable although so small. The characters 

 of the body-chamber and aperture have not been observed. The septa are nearly 

 direct, and distant ^ the diameter ; their convexity is f- the same line. The siphuncle 



