SILURIAN MOLLUSCA. 181 



has the long (antcro-posterior) diameter at tlie large end 1 inch 

 4 lines, at the small end 9 lines ; short (lateral) diameter at 

 large end 10 lines ; length of last chamber 1 inch 1 line ; the 

 last five or six septa 1^ line apart in the middle of the side. 



1 have not clearly seen the siphon of this species, which is 

 about as thick and slightly arched as the P. arcuatum (of which 

 it is figured as a separate variety by Sowerby), but tapers much 

 more slowly as in the P. compressum. 



Not uncommon in the green mudstone (Lower Ludlow rock) 

 of Green quarry, Leintwardine. 



{Col. University of Cambridge, &c.) 



Cycloceras tenui-annulatum (M'Coy). 



Sp. Char. Nearly cylindrical (tapering half a line in 2 inches at 

 a diameter of 6 lines) ; rings narrow, sharply defined, half a line 

 wide, slightly oblique, six in half an inch at the above dimen- 

 sions ; surface with very minute, longitudinal, equal strise, 

 twelve or fourteen in a space of 1 line; towards the small 

 end a few circular strise on each ring decussating the longitu- 

 dinal lines. 



This species differs constantly from the Orthoceras {Cycloceras) 

 Ibex by the narrower and more sharply defined rings, and their 

 considerably greater number in a given space in specimens of the 

 same size ; the longitudinal striation is even finer than in that 

 species. 



Not uncommon in the green Lower Ludlow mudstone of Green 

 quarry, Leintwardine, and near Aymestry ; rare in the Upper 

 Ludlow quartzite of Brigsteer, Kendal. 



{Col. University of Cambridge.) 



Orthoceras politum (M^Coy) . 



Sp. Char. Very elongate, conic, regularly tapering at the rate of 

 half a line in 1 inch, from a diameter of 7 lines ; from which 

 size to 5 lines, the septa have a uniform distance of 3 lines 

 apart ; they are slightly oblique, convex, with even edges ; 

 siphon moderate, excentric, its own diameter from the centre ; 

 surface smooth. 



One specimen with a glossy, horn-like external surface, slightly 

 imperfect at each end, measures 1 inch 4 lines in diameter at the 

 mouth, is 1 foot 8 inches long, and measures 2 lines in diameter 

 at the smaller end, where the septa are slightly oblique, and 

 l~ line apart. 



Not uncommon in the impure calcareous concretions of Glen- 

 quapple, Scotland. 



{Col. University of Cambridge.) 



