260 CARBONIFEROUS LIMESTONE FOSSILS. 



by the disparity in width of the parts of the whorls above and 

 below the band, and the disparity in number of the keels or 

 spiral strise which ornament those parts. 



Not very uncommon in the impure carboniferous limestone of 

 Lowick, Northumberland. 



(Col. University of Cambridge.) 



Pleurotomaria decipiens (M^Coy). 



Desc. Var. ol. Spire acute, regularly conic ; apical angle about 

 40° ; composed of about seven or eight gradually increasing- 

 flat whorls ; suture a simple impressed line ; base of body- 

 whorl flattened, forming a strong angle with the spire; no 

 umbilicus ; pillar-lip slightly thickened, arched ; mouth ob- 

 long angulated, a little wider than long ; surface marked with 

 narrow thread-like spiral ridges, separated by flat or slightly 

 concave spaces about three times their width ; about eight or 

 nine «piral ridges on each whorl, and seven or eight more 

 rather stronger on the base, the intervening spaces very irre- 

 gularly cancellated by obscure, unequal, obtuse, longitudinal 

 wrinkles, very slightly oblique to the band, forming an obscure 

 irregular, quadrate pitting, occasionally visible on the cast, 

 these transverse plicae closer and more oblique on the base ; 

 band about the width of the ordinary spaces between the spiral 

 lines, flat, very inconspicuous, and bounded by very delicate, 

 impressed lines, destitute of the obtuse cancellating plicae of 

 the rest of the surface, situated two inter-spiral spaces above 

 the lower suture of each whorl. Width 10 lines, proportional 

 height of last whorl -Y^^y height of penultimate whorl j^^*^, 

 width of mouth y^^. 



Var. 13. Very elongate conic ; apical angle about 40^ ; whorls 

 moderately convex ; base of basal whorl gradually prolonged, 

 not flattened nor separated by an angulation from the sides ; 

 mouth a little longer than wide. Length of last whorl 11 lines, 

 proportional width -fj^-^, height of penultimate whorl y'y^^. 



It will be seen that this species has two extreme varieties 

 somewhat resembling those of the P. yvanni ; the variety a. so 

 exactly resembles a Trochus, that it requires the most careful ex- 

 amination to detect the extremely obscure, though definite band, 

 to convince the observer that it is a Pleurotomaria ; the var. /3, 

 with the basal whorl elongate and rounded in front or at base, 

 like the corresponding variety of P. yvanni, is so like a Macro- 

 chihis, that it is only by carefully tracing the intermediate forms 

 and detecting the very obscure band, noting the same number of 

 spiral ridges on the whorls, &c. that I have become satisfied of 



