206 MOUNTAIN LIMESTONE FOSSILS. 



near the small, rounded, anterior end, in which there is an 

 abruptly hollowed space beneath the beaks ; posterior end 

 narrow, square, truncated ; diagonal ridge angular, slightly 

 sigmoid, strongly defined from the beak to the respiratory 

 angle ; posterior slope smooth, slightly concave ; sides slightly 

 flattened, with coarse irregular striae and irregularities of 

 growth parallel with the margin ; ventral margin nearly 

 straight ; hinge-line as long as the shell, its inflected margins 

 broad, slightly concave, nearly at right angles to the plane of 

 the margin of the valves ; cardinal ridge thick, obtuse, diver- 

 ging nearly half the width of the posterior slope from the hinge- 

 line. Length 1 inch 4 lines, proportional width from beak yYo> 

 width at posterior end y-^^^, length of anterior end y\y%, depth 

 of one valve (greatest about the middle of the diagonal ridge) 



100* 



This rare species is remarkable for its narrow square posterior 

 end and strong angular diagonal ridge. It is proportionally 

 shorter and less regularly ridged than the S. angustatus (Phill.). 

 Goldfuss's figure under this latter name nearly agrees with our 

 shell and is no doubt identical. 



In the impure carboniferous limestone of Lowick, Northum- 

 berland. 



{Col. University of Cambridge.) 



Sanguinolites variabilis (M'Coy). 



Sp, Char, Oblong, tumid, nearly closed at the ends ; averaging 

 twice as long as wide (sometimes a little more, sometimes a 

 little less) ; beaks large, tumid, oblique, close to the anterior 

 end, which varies from one-ninth in large, to one-seventh of 

 the length in smaller specimens ; anterior lunette large, smooth, 

 oval, contracting the round anterior end; ventral margin nearly 

 straight, or commonly with a wide shallow sinus, very rarely 

 with a slight convexity ; valves evenly tumid or with a slight 

 broad mesial concavity or flattening ; posterior slope flattened, 

 smooth, defined by a diagonal slightly sigmoid ridge, sharp 

 and angulated near the beak, gradually becoming rounded 

 and obscure towards the inferior posterior (respiratory) angle 

 in old specimens ; greatest depth of the valves along this line 

 and about half-way from the beak ; hinge-line with a slight up- 

 ward curvature, posterior lunette very wide, concave, nearly 

 horizontal ; surface marked by concentric wrinkles, variable in 

 size and strength, usually thickening slightly on reaching the 

 diagonal posterior ridge, almost always undulated and irre- 

 gularly interrupted about the middle and anterior third of the 

 sides (averaging five or six in the space of 3 lines from the 



