﻿106 BKITISH FOSSIL CEPHALOPODA. 



General Description. — The diameters are not seen nearer to equality than the 

 ratio of 18 to 13. The rate of increase is too variable to measure, but it is not 

 greater than 1 in 14. The ornaments are peculiar ; in the earlier portion they are 

 really grooves with rounded edges, and from their abundance look like striee, but as 

 these become wider and closer the intervening parts take the character of almost 

 sharp but not separate ribs, of great irregularity. They have an obliquity some- 

 times amounting to 12° in larger specimens, but appear to undulate only in small 

 ones. Their distance is variable, from -^ to ^ the diameter, or even less ; they have 

 an irregular imbrication. The septa are direct, and have a convexity of -^ the long- 

 diameter ; they are distant f- the diameter : the last chambers appear to be closer 

 than the rest ; the siphuncle is central. The greatest length seen is 8j inches, and 

 the greatest diameter 1^- inches. I consider a number of small specimens coming from 

 the same locality as the young of this species ; they have grooves or strise for their 

 ornaments, distant £ of a line, one specimen showing such grooves changing into 

 intervals between the ribs : some examples show a fracture and repair during life. 



Relations. — The nearest species to this is 0. Duponti, which has its ribs somewhat 

 at the same distance and also irregular, but in the present they are scarcely true 

 ribs, but rather irregular transverse folds, on which finer ornaments would never be 

 sought. Such roughness induces me to associate the specimens (figs. 7, 8) from 

 Ledbury with this, though much nearer to 0. Duponti than the Scotch specimens are. 



Distribution. — The typical examples are all from the Upper Silurian Beds at Esk 

 Eeservoir, in the Pentland Hills (8), whence also come the examples referred to the 

 young of this (17). It occurs also on the west side of Kirkcudbright Bay (1), and 

 apparently in the Upper Ludlow of Helmfoot (1) ; also in the Lower Ludlow of 

 Ledbury (3). Salter records it also from Lesmahagow. 



Section Angulati. 



Orthoceras angulatum, Wahlenberg, PI. VII. figs. 1, 3, 4, 8, 9. 



1821. Orthoceras angulatum, Wahlenberg, 'Nova Acta Reg. Soc. Sci. Acad. Upsala,' 



vol. viii. p. 90. 

 1852. „ „ M'Coy, ' Palaeozoic Fossils,' p. 313. 



1854. „ „ Morris, ' Cat. of Brit. Fossils,' p. 309. 



Syn. 1839. Orthoceras virgatum, Sowerby in Murchison's ' Silurian System,' tab. 9, fig. 11. 

 1839. „ canaliculatum, Sowerby, loc. cit., tab. 13, fig. 26, p. 632. 



Query 1837. Orthoceras angulatum, Hisinger, ' Letb. Suec' p. 28, pi. 10, fig. 1. 



Not 1866. Orthoceras angulatum, Barrande, ' Syst. Sil. de Boheme,' pi. 233, p. 692. 



Type. — The type is not figured by its author, though by universal consent he 

 referred to a species identical with our British form. His description is, " The 



