ATHYRIS 469 



ties a little rounded. The dimensions of the type specimen are: length 

 17.5 mm., width 28 mm., length of hinge-line 27 mm., thickness 12.5 mm. 



Pedicle valve moderately convex, the greatest convexity posterior to 

 the middle, the surface curving abruptly from the umbonal region to the 

 cardinal margin, sloping in nearly straight lines to the cardinal extremi- 

 ties and curving gently to the antero-lateral margins ; the beak contiguous 

 with that of the opposite valve, apparently pierced by a subcircular fora- 

 men; cardinal area narrow and somewhat ill-defined; the characters of 

 the delthyrium not determinable upon the type specimen, but it is prob- 

 ably broadly triangular as in A. lamellosa; lateral slopes of the valve 

 convex; mesial sinus deep and sub-angular in the bottom, originating 

 at the beak, rather sharply defined laterally. 



Brachial valve about equally convex with the pedicle, the greatest con- 

 vexity near the middle, the surface curving gently to the cardinal margin 

 except for a short distance on each side of the beak where it is more 

 abrupt, the curvature to the antero-lateral margins is gentle, and the 

 surface is somewhat compressed towards the cardinal extremities; the 

 beak incurved beneath that of the opposite valve ; the mesial fold origin- 

 ating at the beak, flattened on top and becoming somewhat abruptly 

 elevated towards the front of the shell, apparently with a longitudinal, 

 mesial depression anteriorly. 



Surface of both valves marked by concentric lamella? from one to two 

 millimeters apart on the central portion of the shell, becoming more 

 conspicuous and somewhat crowded towards the anterior margin. 



Remarks. The description of this species has been made from a very 

 perfect sulphur cast of the type specimen. The type itself has been lost 

 and no other example of the species has been seen by the writer. It differs 

 from A. lamellosa in its smaller size, its greater proportional width with 

 the greatest width near the hinge-line, its much longer hinge-line and 

 its much stronger fold and sinus. The concentric lamellae of the shell 

 are represented by their basal portions only, but when perfectly preserved 

 they doubtless were produced into lamelliform extensions similar to 

 those on A. lamellosa, but much thinner and closer together. 



H orizon. Chester group. 



ATHYRIS DENSA Hall 

 Plate LXXVII, Figs. 60-69 



1882. Centronella crassicardinalis Whitfield, Bull. Am. Mus. Nat. Hist., 



vol. 1, p. 55, pi. 6. figs. 50-52 (not Athyris crassicardinalis White, 

 1860). 



1883. Centronella crassicardiiwlis Hall, 12th Ann. Rep. Geol. Surv. Ind., 



pi. 29. fies. 50-52. 



1894. Athyris densa Hall, 13th Rep. N. Y. State Geol., p. 651. 



1895. Athyris densa Hall and Clarke, Pal N. Y., vol. 8, pt. 2, p. 364, 



pi. 46, figs. 6-32 



