﻿172 BRITISH FOSSIL CEPHALOPODA. 



Relations. — A magnificent species figured by Barrande (' Syst. Silur. de Boheme,' 

 pi. 483, 484), under the name of Cyrtoceras turnus, has a siphuncle of similar 

 characters to the present, viz. its narrowing between the septa and its longitudinal 

 lines, and a side view of the shell is similar in general appearance. The siphuncle, 

 however, is close to the convex side, and attains a diameter of only -£$ instead of 

 | the whole diameter of the septum, while the section of the shell is much more 

 transverse. The sutures, moreover, are perfectly straight, and not sigmoidal. 

 Although, therefore, they belong to the same group, these differences seem worthy 

 of recognition by giving a distinct name for the present form. Barrande's species 

 comes from his stage Gr, or highest part of the Upper Silurian rocks, whereas ours 

 comes from the lowest. 



Distribution. — In the Upper Llandovery rocks of Craig-yr-Wyddon (1). 



Section Exogastrica. 



Cyrtoceras approximatum, Sowerby, PL XX. figs. 1, la. 



1838. Orthoceras approximatum, Sowerby in Murchison's 'Silurian System,' pi. 21, fig. 22. 

 1854. Cyrtoceras approximatum, Morris, ' Catalogue of British Fossils,' p. 302. 



Type. — The section is circular. The rate of increase is very slight at the smaller 

 end, and almost zero at the larger. The curvature is slight, having a radius of 

 7^ inches. The surface is only seen by the external cast. The appearances 

 presented are rather contradictory. In the part preserved the septa appear to be % 

 the diameter apart, but on the external cast there are lines which are twice as close ; 

 these may be interpreted either as ribs or septa. The apparent remoteness of the 

 latter on the upper part would lead to the former, but another specimen suggests the 

 latter, interpretation. The convexity is moderate, and the siphuncle minute and 

 external. From the Upper Llandovery Sandstone of Eastnor Park. In the Museum 

 of the Geological Society. 



General Description. — A better preserved example, apparently referable to this 

 (fig. 1 ), shows a section with equal axes, but scarcely circular ; the increase is greater 

 on approaching the inflated body-chamber, which is -f its basal diameter in length, 

 and the aperture has a simple circular outline. The curvature is almost lost. The 

 septa in this are certainly i the diameter apart, but no siphuncle can be found on 

 the rather conically-shaped septal surface (fig. la). 



Relations. — The nearest species to this in shape is Orthoceras f return, but there 

 is no sign of curvature in that, and the body-chamber is different. 



Distribution. — In the Upper Llandovery, Eastnor (1), and in Wenlock Beds (?)> 

 Dudley (1). 



