﻿BRITISH FOSSIL CEPHALOPODA. 229 



interspaces being uniform concavities between the crests. Salter would appear to 

 have seen transverse striae on it. Length, 2| inches ; greatest diameter, ^ inch. 

 From Ludlow Beds, Black Mountain, Clun Forest. 



General Description. — The section, as Salter states in the description of his 

 0. perelegans, is probably circular. The flattening always takes place in the plane 

 of curvature. The radius of curvature is in none so small as in the type. In the 

 smallest, almost reaching the apex, it is -f inch, and the curvature gradually decreases 

 as the fragments are of larger diameter — and those which show the aperture are 

 nearly straight there ; some also may have longer straight portions. The earlier 

 portion forms an open coil, the whorls not being in contact. The greatest rate of 

 increase observed is 1 in 9, and this decreases with the curvature to almost zero. 

 The aperture is not contracted, but formed by a sigmoid curve which bends rapidly 

 forward on the convex side as to a beak ; it is concave forwards on the side, and curves 

 back to form a sinus on the inner side. The ornaments consist of, first, sharp non- 

 separate ribs, at first \ the diameter apart, becoming closer to an average of ^ the 

 diameter, though appearing still closer by compression, and finally dying off on the 

 body-chamber on approaching the aperture — these rather undulate, or are oblique, 

 sloping backwards to the exterior ; secondly, there are fine riblets parallel to these, 

 numbering from 10 to 20 in the interval between two ribs, and degenerating into 

 lines of growth on the unribbed part. The septa lie parallel to the ribs in the 

 intervals between them, and are thus about ^ the diameter apart ; their convexity 

 is between ^ and ^ the diameter. The siphuncle is small and central. The type is 

 the longest species. The greatest diameter of the more curved part is f- inch. 



Relations. — Although, as we have seen, some Orthocerata and a Cyrtoceras have 

 very similar, though not identical ornaments, yet the changes of the curvature which 

 take place in this are of sufficient importance to separate it from them all ; and if the 

 large species doubtfully referred to the present genus or to Trochoceras should be 

 assigned to the latter, this would be the only true Lituites in the British fauna. 

 However straight the last part may be, there is seldom wanting some indication 

 of its true character. 



Distribution. — In the Lower Ludlow of Ledbury (6) and of Leintwardine (3) ; 

 in the Upper Ludlow of Ludlow (4), Malvern (3), Kirby Moor (1), and Under- 

 barrow (1) ; and in the Tilestone of Horeb Chapel (1) and Llandeilo (1). 



