﻿96 BRITISH FOSSIL CEPHALOPODA. 



extreme length, viz. about ten times as long as its basal diameter, and this will 

 account for the numerous examples showing no trace of septa. The aperture is 

 simple, parallel to the ribs, and without contraction or expansion. The ornaments 

 consist of rounded or subangular, characteristically separate ribs, showing some 

 tendency to imbricate downwards ; these are distant in very varying proportions, 

 ranging from ^ to f- the longer diameter ; the distance is seen to change in the 

 length of a single specimen, being least at the base of the body-chamber; they have 

 an obliquity amounting in some to 10° : that this is sometimes seen on the broader 

 side, and sometimes is only indicated there by the undulation of the ribs, is due to 

 the different directions of compression. The surface, when well preserved, has 

 numerous sharp transverse lines parallel to the ribs, generally too fine to count : 

 these are often absent, and are considered in that case to have been worn away : 

 among such are the two contracting body-chambers ; but no longitudinal lines are 

 ever seen in those referred to this species. Towards the aperture, as seen in some 

 examples, the ribs die away over a distance of about 2 inches, the last preserved 

 becoming enfeebled, but the finer lines continue over this surface. In none of the 

 specimens examined, which show this change on approaching the larger end, do any 

 septa appear at the smaller, and in one (fig. 4) there are 5 inches without any septa 

 being seen ; hence these specimens, except the last, may be considered as showing 

 only the final half of the body-chamber. The septa, when seen, lie in the hollows 

 between the ribs, one to each ; but the septum is not quite parallel to, but more 

 direct than, the ornaments; the surface has a convexity of ^ the corresponding 

 diameter. The siphuncle, of moderate and uniform size, lies on the long dia- 

 meter, at a distance of f of that line from the side to which the ribs slope back. 

 The greatest length seen is 7| inches, and the long diameters are from 9^ lines to 

 4^ lines. The specimens from the north of England have a greater angularity and 

 more obliquity in the ribs than those from the typical Silurian districts, but differ in 

 no other respect. The rapid expansion seen in fig. 5 is noticeable, as it is a feature 

 which seems to be commoner in this species than in any other, and on this ground 

 we may refer to two curious examples — one from the Wenlock Limestone, and the 

 other from the " Upper Llandovery " beds of Shallallymore — in the Museum of 

 Practical Geology, which show a very sudden change, the upper part being at the 

 same time almost ribless. 



.Relations. — The character of the ribbing, especially in southern examples, agrees 



with that of 0. tenuiannulatum in being rounded and separate, but the finer orna- 

 ments are transverse. Those examples which show none of these are placed with 

 the present species, on account of the greater ease with which transverse lines may 

 be lost. From 0. tracheale this differs by the obliquity of its ribs, and from Cyrto- 

 ceras ibex, by its want of curvature. The 0. Hisingeri of Boll, founded on Hisinger's 



0. annulatum, was identified by Sowerby as belonging to his species, and my 



