﻿104 BRITISH FOSSIL CEPHALOPODA. 



Distribution. — It occurs in the Coniston Flags of Helm Knot (1) ; in the Lower 

 Ludlow of Kingston (4), Radnor Forest (1), and Leintwardine (2) ; in the Upper 

 Ludlow of Brigsteer (1), and in the Upper Silurian on the west side of Caherconree, 

 co. Kerry (6). 



It is recorded by R. Etheridge, jun., from the Upper Silurian of the Pentland 

 Hills, and in the Catalogue of Western Scottish Fossils from Old Red Sandstone ( ! ), 

 Carmichael Burn, Lanark. 



Orthoceras Etheridgii, Blake, PI. YI. figs. 3, 4, 5, 5a, 6, 6a. 



Type. — The section is nearly circular, the ratios of the diameters being 18 to 17 ; 

 the rate of increase for the longer diameter is 1 in 10. No details of the body- 

 chamber or aperture have been observed. The ornaments consist of transverse 

 impressed grooves, the bases of which are flat and the interspaces flat also, and three 

 times the width of the grooves. These are undulating and oblique, 13° on the broader 

 side, and at a somewhat inconstant distance, about -^ the diameter, at the larger 

 end. The septa are more direct than the ornaments, and have a convexity of ^ the 

 long diameter ; their distance is § of the same line. The siphuncle is about -g- the 

 diameter, and lies f of the diameter from the side to which the ornaments slope back. 

 The remarkable feature of this species is that, taking the place of the septal surface, 

 there is a peculiar inflated surface which is more or less continuous with the outside 

 of the shell ; over the siphuncle and leading down to it is an elongated deep hollow, 

 in the direction of the longer diameter ; from this, radiating impressed lines or 

 furrows proceed to the circumference, having the aspect of being produced by 

 folds. The length is 7^ inches, and the greatest diameter 1^ inches. From the 

 Upper (?) Silurian on the east side of Kirkcudbright Bay, where it was found 

 by R. Etheridge, jun., after whom I have named it. In the Museum of the 

 Geological Survey, Edinburgh. 



General Description. — The other examples found at the same time and sub- 

 sequently, by my friend R. Etheridge, jun., one of which he has bad cut in a 

 longitudinal section, confirm the remarkable features of the above. The section is 

 not so nearly circular at the smaller end, but the rate of increase is pretty constantly 

 as in the type. The grooves are seen to vary somewhat in their distance, the 

 maximum being \ the diameter. The section shows that the siphuncle narrows at 

 the junction of the septa and expands cylindrical ly in the chambers ; the septal 

 distance and convexity is confirmed, and it is seen that the surface which is exposed 

 at the ends is not the septal surface ; the latter are apparently smooth and the thick- 

 ness small, and the short necks are seen to turn rapidly outwards from the siphuncle ; 

 above these is the dark deposit, whose exterior is exposed when the fossil becomes 

 broken ; this has a greater convexity than the septum, and is continuous in appearance 

 with the exterior of the shell. Its occurrence in two or three chambers proves 





