SPIRIFERELLA 413 



Description. Shell large, subglobose, wider than long, the hinge-line less 

 than the greatest width of the shell, the cardinal extremities rounded. 

 The dimensions of a nearly perfect individual are : length 56 mm., width 

 70 mm., thickness 48 mm., length of hinge-line 51 mm., height of cardinal 

 area 11.5 mm. 



Pedicle valve ventricose, the greatest convexity in the umbonal region, 

 the surface curving abruptly in all directions from the umbonal region 

 but most abruptly to the cardinal margin, the convexity of the valve ex- 

 tending out to the cardinal extremities ; the beak not sharply pointed, a 

 little incurved ; cardinal area rather high, arched, the inferior portion 

 making an angle of about 90 degrees to the plane of the valve, the curva- 

 ture from the hinge-line to the beak describing nearly an arc of a circle, 

 the lateral margins not sharply defined but rounding rather abruptly into 

 the lateral slopes of the valve, the delthyrium large, forming nearly an 

 equilateral triangle ; lateral slopes of the valve each marked with from 16 

 to 20 simple, depressed, rounded plications, the smaller of which towards 

 the cardinal extremities become nearly obsolete ; mesial sinus narrow and 

 shallow at the beak, growing rapidly broader towards the front, of mod- 

 erate depth, rounded in the bottom, without plications, in full grown 

 examples with the anterior extremity produced into a broad, lingual 

 extension. 



Brachial valve as convex or more convex than the pedicle, its greatest 

 convexity usually at the anterior margin, the convexity extending to the 

 cardinal extremities or rarely slightly compressed at the extremities ; the 

 heak incurved and the umbonal region projecting beyond the hinge-line ; 

 the mesial fold without plications, scarcely defined at the beak, becoming 

 broad and high and rounded anteriorly; the lateral slopes with a steep, 

 convex slope from the margins of the mesial fold to the margins of the 

 valve, the slope becoming more abrupt to the cardinal margin, each bear- 

 ing plications similar in form and number to those upon the opposite valve 

 except that they are often a little more distinct. 



The finer surface markings consist of exceedingly minute papillae or 

 granulations more or less rhombic in outline, with the longer axis longi- 

 tudinal and slightly elevated anteriorly, arranged in more or less definite 

 diagonal lines. More or less irregular concentric lines of growth are 

 usually present which are sometimes crowded towards the anterior 

 margin. 



Internally the shell is characterized by the exceedingly elongate dental 

 lamellae in the pedicle valve, which extend with only moderate divergence 

 nearly or quite one-half the length of the shell from the beak towards the 

 anterior margin, these lamellae are thick and bifoliate, splitting readily 

 along their median plane; posteriorly a concave transverse plate joins 

 the two dental lamella? in a plane a little depressed below the general 



