﻿BRITISH FOSSIL CEPHALOPODA. 93 



The aperture is slightly inclined in the direction of the ornaments, and there is a 

 constriction below it. The ornaments change with growth. In the earlier part of 

 the shell they are rounded, low rings, wider than the interspaces, 8° oblique, and 

 distant ^-^ the diameter; at a certain period of life, long before the septa have 

 ceased to be formed, these rings disappear rather rapidly and very completely. These 

 larger ornaments are covered by sharp, upright parallel riblets, about 14 in the 

 space of a rib ; these are continued after the ribbing has ceased, and disappear 

 themselves at last. The septa are direct, and quite independent of the ornaments ; 

 their distance is exceedingly variable, rapidly changing from f to ^ the diameter, 

 and their convexity decreases at the same time. The siphuncle is f- across the 

 diameter, its own diameter is ^ the same on the septal surface, and its elements are 

 slightly inflated. The longest fragment seen is 8 inches, and its diameter 2 inches. 

 It occurs chiefly in the band E 15 but also in E 2 of the Upper Silurian. 



General Description. — It is obvious that both smooth and ribbed fragments may 

 equally belong to this varying shell, but a reference to it will only be justified when 

 one is seen passing into the other. The most characteristic example is that figured 

 in fig. 1. It is flattened, and so the section and true tapering are unknown ; the 

 latter appears to be 1 in 6 ; at the lower end there are low undulations, irregular in 

 size and distance, on the average \ the flattened diameter, and 8° oblique ; these are 

 seen entirely to die away for 1^ inches, though the septa have not ceased. In the 

 other example figured the ribs are direct ; but as all other characters agree, this 

 difference may be due to the directions of the compression. The whole is covered 

 by sharp, upright riblets, from 9 to 15 to each rib space, which are continued 

 after the cessation of the ribs. The septa are direct, have a considerable convexity, 

 and the siphuncle is nearly central. The greatest length is more than 6 inches by 

 § inch diameter ; one example shows the shell to have been broken and restored 

 during life. 



Relations. — The younger portions of these shells might very well be referred to 

 Minister's 0. subannulare, as far as his description goes, but that name has been re- 

 stricted, after Barrande, to those with less numerous upward imbrications, the riblets 

 in these being uniform and upright. It is thus to the present species rather than to 

 Minister's that the specimens referred to by Salter, and partly by M'Coy, should be 

 referred. Another difference between the two is that in 0. subannulare there is not 

 so great a change in the characters towards the aperture. From 0. dulce this 

 differs by the low, merely undulating character of its ribs. Nevertheless it is by no 

 means impossible that those with more direct and more clearly marked ribs ought to 

 be referred to the latter species. 



Distribution. — In the Wenlock Shale of Oarneddau, Builth (1) ; in the Lower 

 Ludlow of Ledbury (3), Aymestry (1), Leintwardine (1), Elton (3), Dudley (3), and 

 Wenlock (1). 



