﻿BRITISH FOSSIL CEPHALOPODA. 141 



Some young forms show a little curvature (fig. 6). The rate of increase lies between 

 1 in 9 and 1 in 13, and in the largest specimen, figured by Salter, is reduced to 

 1 in 22, and in none does it increase to 1 in 5 or 6. The body-chamber is not 

 certainly known, as the specimens last referred to may be all septate. The exterior 

 is seldom preserved, and shows only lines of growth. The septa have considerable 

 convexity, viz. from -^ to f the diameter ; they are slightly undulating and oblique ; 

 their distance apart constitutes the peculiar feature of the species, as it is essentially 

 variable : on the same specimen it changes from ^ to § the diameter, and in others 

 from 1 to 2 diameters ; the distance is not, however, constant for more than 2 septa, 

 the average being more than ^ the diameter. The siphuncle is small, and situated 

 on the most convex part of the septum, or about central in a transverse diameter. 

 On one specimen (fig. 5) are two oblique grooves, ^ inch apart, of trapezoidal 

 section, as in 0. Etheridgii; and in another some curvature is shown towards the 

 apex, which exposes the cicatrix very clearly ; none of these have more than 

 ^ inch diameter. The greatest diameter seen is 1^ inches, and the longest 

 specimen measures 5 inches. 



Relations. — The variability of the distance of the septa does not justify the 

 inclusion of specimens containing moderately close septa throughout, and this 

 character separates most species. In 0. distans the septa, though distant, are more 

 regular, and the siphuncle is excentric. This latter character differentiates also 

 O.politum. The quadrate section is also a peculiarity of the present species. The 

 name 0. remotum appears to have been used by Salter in the first instance, under 

 which name the species is quoted by Baily, but subsequently it was changed. 



Distribution. — The smaller forms are found in the Lower Llandeilo flags of 

 Cefn Grwynlle, Shelve (12), but the majority are from the Bala Series at Rhiwlas, 

 Bala (18); they occur also at Troutbeck (1), Dufton (1), and Coniston (1), and 

 perhaps at Desertcreat (1), and the Chair of Kildare (2). 



Harkness and Nicholson record it also from the Keisley Limestone, and Baily 

 mentions it from the Lower Silurian of Portraine. Its occurrence in Spain was 

 noticed at its first establishment. Perhaps Barrande's 0. curvens represents the 

 same species in Bohemia. 



Orthoceras politum, M'Coy, PI. IX. figs. 1, 2, 8. 



1851. Orthoceras politum, M'Coy, ' Ann. Nat. Hist.' Series ii. vol. vii. p. 45. 



1851. „ „ Salter in Murcbison, ' Quart. Journ. Geol. Soc' vol. vii. p. 137, 



pi. 10, figs. 5, 6. 



1852. „ „ M'Coy, ' Palaeozoic Fossils,' pi. 1, fig. 30, p. 316. 

 1865. „ „ Haswell, ' Silurian Kocks of the Pentlands,' pi. 1, fig. 1. 

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



Syn. 1846. Orthoceras acuarium, M'Coy, ' Sil. Foss. Ireland,' p. 7 (not of Miinster). 



