﻿110 BRITISH FOSSIL CEPHALOPODA. 



Distribution. — In the Lower Silurian rocks, Leenane (2) and Maume (1) ; and in 

 Bala Beds, Piedmont Glen, Ayrshire (1), Girvan (3), and GlengrafT (1). The 

 example from the latter locality is in the Museum of Practical Geology. M'Coy also 

 records it from Blackwater Bridge, and Clifden, co. Galway, and Kelly from Bunowen. 



Orthoceras originale, Barrande, PI. VII. figs. 5, 5a, 10. 



1868. Orthoceras originale, Barrande, 'Syst. Silur. de Boheme,' pi. 267, p. 206. 

 Syn. 1868. Orthoceras striato-punctatum, Barrande, loc. cit., pi. 268. (Not of Minister.) 



1865. „ dulce, Salter, ' Catalogue of Fossils in the Museum of Practical Geology.' 



(Not of Barrande.) 



Type. — The section is circular. There is a little curvature in long shells. The 

 average rate of increase is 1 in 6, but specimens are figured in which it varies 

 between 2 in 15 and 2 in 7. The body-chamber is five times its basal diameter, and 

 very little change is seen towards the aperture. The ornaments consist of regular 

 impressed lines, without fresh ones introduced, about 60 in number. The base of 

 these lines is smooth in typical examples ; the intervening spaces are quite flat and 

 smooth, but towards old age the right-hand side is more elevated, and the surface 

 has extremely fine longitudinal strife, also some transverse hollow lines declining 

 towards the right side. Occasionally one band has similar but deeper transverse 

 lines at an earlier age. On the inner layer of the shell there are raised lines to 

 meet the impressed strige, and concavities to correspond to the intermediate flat 

 bands, so that the whole shell is formed of parts which in section look like the 

 links of a chain. The concavities are often finely striated transversely. The septa 

 are horizontal, and have a convexity of \ and a distance of f the diameter. The 

 siphuncle is nearly central ; it has a diameter y 1 ^- of the whole diameter, and is 

 contracted at the septa. The largest specimens have a diameter of 1^ inches, and 

 a length of 7 inches. It is a widely distributed species, commencing in Ds, or Lower 

 Silurian, and passing through E2 to Fi of the Upper Silurian. 



General Description. — The peculiar feature of this shell, which it shares with the 

 fossil referred by Barrande to Miinster's 0. striato-punctatum, but which does not 

 agree with that author's description of his shell, is the reversed character of the 

 inner and outer layers, the furrows of the latter coinciding in position with the 

 raised lines of the former. This character is well seen in several British examples 

 (see fig. 5a). These are usually flattened in the shale, and are otherwise imperfect ; 

 their true section is not therefore well seen, but appears to be circular. The rate of 

 increase is very variable, according to the pressure, ranging thus between 1 in 12 

 and 1 in 5. The body-chamber is not certainly distinguishable ; the aperture had (?) 

 a sigmoidal outline. From 48 to 60 longitudinal sulci may be counted which have 

 not been observed to have any punctures; the spaces are sometimes obscurely 



