﻿BEITISH FOSSIL CEPHALOPODA. 153 



Relations. — The general shape is nearest to 0. gregarium, but no other species 

 match it in the flatness of the septa. 



Distribution. — In the Wenlock Shale of Radnor (1), of Clytha, Usk (1), and of 

 Gorstley (1 ?) ; and in the Upper Ludlow of Ludlow (1). 



It is also recorded by Sowerby from the Lower Ludlow of Radnor ; by Phillips 

 from Woolhope, and by Brown and Henderson from the Upper Silurian (bed D) of 

 the Pentlands. 



Orthoceras imbricatum, Wahlenberg, PI. XI Y. figs. 1, la, 3, 3a, 4, 4a, 5, 6. 



1821. Orthoceras imbricatum, Wahlenberg, 'Nov. Act. Upsala,' vol. viii. p. 89. 

 1827. ,, „ Hisinger, ' Lethsea Suecica,' tab. 9, fig. 9, p. 89. 



1870. „ „ Barrande, ' Syst. Sil. de Boheine,' pi. 440. 



Syn. 1848. Orthoceras marloense, Phillips, 'Mem. Geol. Surv.' vol. ii. pt. 1, pi. 13, fig. 1. 



Not ? 1839. Orthoceras imbricatum, Sowerby in Murchison's ' Sil. Syst.' pi. 9, fig. 2, p. 620. 

 1852. „ „ M'Coy, 'Pal. Foss.' p. 315. 



1873. „ „ Salter, ' Cambrian and Sil. Fossils,' p. 187. 



Type. — Wahlenberg's short description merely states that the type consists of 

 imbricating sheaths, which are very convex, and not more than a line apart; and 

 the siphuncle is very fine and filiform, and is central. The figure given by Hisinger 

 to represent Wahlenberg's shell appears to have misled some who had not referred 

 to the latter author, to suppose the siphuncle was nearly lateral. This figure, 

 however, may nevertheless be taken to also represent the type, especially as its 

 inconsistencies with the description admit of explanation. The section is probably 

 not circular; the rate of increase is 1 in 9. The septa are moderately convex, and 

 have an obliquity, according to the figure, of more than 15°. The siphuncle is too 

 large to be called filiform, and is not quite in the centre. The greatest diameter is 

 1^ inches, and the greatest length, all septate, is 3^- inches. From the Upper 

 Silurian of Gothland. 



General Description. — The British specimens that are fairly referable to this 

 species show considerable variation in many points. The section in some is nearly 

 circular, with a tendency to being quadrate ; but usually it is more or less elliptic 

 in varying proportions, possibly owing to compression. The rate of increase of 

 septal portions is between 1 in 7 and 1 in 9 ; but in the largest examples it is 1 

 in 12 or 13, and in one body-chamber, otherwise inseparable, only 1 in 15 (fig. 1). 

 This body-chamber has a length of 4^ inches, which is 2|- times its basal diameter. 

 There are no constrictions below the aperture, which is not well preserved. There 

 are no ornaments beyond transverse almost invisible lines of growth in the under 

 shell, so that it looks perfectly smooth (fig. la). The convexity of the septa is about 

 two chambers, or f to \ the diameter, the distance varying from y to \ of the same, 

 and the last is sometimes of half-size. The sutures are more or less oblique, and so 



x 



