﻿140 BRITISH FOSSIL CEPHALOPODA. 



4^ inches, and the greatest diameter is § inch. From the Durness Limestone. In 

 the Museum of Practical Geology. 



General Description. — Another example associated with this confirms the general 

 proportions and the distance of the septa, and affords no signs of any ornaments. 



Relations. — It is possible that these are the specimens supposed by Salter to 

 prove that 0. mendax was smooth when young, yet they are larger than other 

 specimens of the latter species which show the ribs, and cannot belong to it if 

 the siphuncle be any guide. It is certainly to be expected that some unornamented 

 species should occur in these rocks. The difference between this and what 0. seri- 

 cenm may be supposed to have been is not great, the chief one being the more rapid 

 increase and more distant septa. The latter character separates it from 0. audax, 

 its nearest ally of later date. 



Distribution. — In the Lower Llandeilo of Durness (2). 



Orthoceras vagans, Salter, PI. XIII. figs. 10, 11, 12. 



1848. Orthoceras vagans, Salter in Sharpe on the Geology of Oporto, ' Quart. Journ. Geol. 



Soc' vol. v. pi. 6, fig. 6, p. 153. 

 1852. „ „ Salter, Appendix A, M'Coy's ' Palaeozoic Fossils,' p. vi. pi. 1 l, 



figs. 28, 29. 

 1852. „ „ M'Coy, 'Pal. Foss.'p. 318. 



1857. „ „ Salter in Murchison's ' Siluria,' Foss. gr. 42, fig. 1. 



1866. „ „ Salter, ' Mem. Geol. Surv.' vol. iii. pi. 24, figs. 1-5, p. 356. 



1873. „ „ Salter, ' Cambrian and Silurian Fossils,' p. 70. 



— „ „ Baily, ' Characteristic British Fossils,' pi. 12, fig. 11. 



Type. — The author's description is : " Smooth ; long tapering when young, more 

 conical when old ; septa broad elliptical, oblique on the longer axis, moderately 

 distant in the young shell, distant by more than the diameter in mid-age, and by 

 less than one-fourth of the diameter when old, deep cup-shaped ; siphuncle nearly 

 central." From the Lower Silurian rocks of Oporto. I have not seen the original 

 specimen, but the figure shows a rate of increase of 1 in 10, and the septa are 

 distant more than the diameter. This figure must be taken as the type, for Salter 

 has, I think, derived his description from several specimens which may not belong 

 to the same species. In particular there is one in the Woodwardian Museum, so 

 labelled by him, which expands at the rate of 1 in 6, and its septa are ^ the mean 

 diameter apart ; there is no proof of this belonging to the same species, but it 

 appears to be the authority for the statements " more conical when old/' and " septa 

 distant less than \ the diameter when old." 



General Description. — The section is more nearly round in small specimens than 

 at a later stage, when it becomes subquadrate and compressed in the ratio of 7 to 6. 



