180 



CHAPTER IX. ^ 

 SILURIAN MOLLUSCA. 



Poterioceras ellipticum (M'Coy). 



Syn. and Ref. Orthoceras pyriforme (Sow.), pars Sil. Syst. 

 t. 8. f. 19 (lower and not upper figure). 



Sp. Char. Elliptical, last chamber conoidal; greatest width at 

 last septum, from whence the chambered and unchambered 

 portions taper elliptically to the contracted mouth and atte- 

 nuated extremity ; septa nearly horizontal, the last three or 

 four about %\ lines apart : greatest width of last chamber (at 

 septum) 3 inches 3 lines ; length of last chamber 2 inches 4|^ 

 lines. 



There are clearly two species confounded by Sowerby in the 

 ' Silurian System ' under the name Orthoceras jjyriforme ; the 

 difference in form he supposed to be produced by the direction 

 of pressure, but I find it to be constant in perfectly uncrushed 

 specimens. To that represented by his upper figure I would 

 restrict his specific name pyriforme, its characteristic pear-shaped 

 form being mainly owing to the greatest width being in the 

 middle of the last chamber, or midway between the last septum 

 and the mouth ; the upper half of the last chamber being abruptly 

 rounded, while the other portion of the shell tapers gradually. 

 In the other species the greatest width is at about the last one 

 or two septa, from whence the last chamber tapers gradually to 

 the mouth with about the same curve that the chambered portion 

 tapers towards the apex, giving a very different, regularly elliptical, 

 figure to the present species, which I have named accordingly. 



Common in the Lower Ludlow rock near Aymestry. 



(Col. University of Cambridge, &c.) 



Phragmoceras intermedium (M'Coy). 



Syn. and Ref. P. arcuatum (Sow.), pars Sil. Syst. t. 11. f. 1. 

 (not t. 10. f. la). 



Sp. Char. Slightly arched, tapering at the rate of 4 lines in 1 

 inch ; section ovate, sides gently convex, outer and inner faces 

 rounded : a specimen (not quite perfect) 2 inches 5 lines long 



* The species in this chapter date from the ' Annals ' for January 1851, 



