ATHYRIS 467 



tance from its anterior margin to the anterior margin of the shell, poste- 

 riorly from the muscular scar, the cast of the transversely striated pedicle 

 cavity is sharply differentiated, its length being about one-half that of 

 the muscular scar ; the dental lamellae^, represented by narrow slits in the 

 cast, extend forward to the anterior margin of the pedicle cavity. In the 

 casts of the brachial valves the cast of the passageway through the pos- 

 terior margin of the hinge-plate to the cavity of the shell is a conspicuous 

 feature, being represented as a median, finger-like process extending to- 

 wards the beak of the opposite valve in a direction nearly at right angles 

 to the plane between the two valves. 



The species is a common and only moderately variable one. The differ- 

 ent appearance of the concentric markings of the shell is frequently due 

 to differences in condition of preservation, but they also vary to some ex- 

 tent in their distance apart. Different examples exhibit some variation 

 in the proportional length and width, but they are frequently crushed so 

 as to make accurate measurement impracticable. 



Horizo-n. Upper Kinderhook, Burlington limestone and Keokuk lime- 

 stone. 



ATHYRIS HANNIBALENSIS (Swallow) 

 Plate LXXVIII, Figs. 10-14 



1860. Spirigera Hannibalensis Swallow, Trans. St. Louis Acad. Sci., vol. 1, 



p. 649. 

 1865. Spirigera Hannibalensis Winchell, Proc. Acad. Nat. Sci. Phil., p. 118. 



1894. Athyris hannibalensis Keyes, Mo. Geol. Surv., vol. 5, p. 91, pi. 41, 



fig. 9. 



1895. Athyris Hannibalensis Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, 



pi. 46, figs. 13-15. 



1908. Athyris hannibalensis Rowley, Mo. Bureau Geol. and Mines, vol. 8, 

 ser. 2, p. 86, pi. 18, figs. 28-30. 



Description. Shell of medium size, subelliptical or approaching sub- 

 circular in outline, the valves moderately and subequally convex, the 

 length usually about seven-eighths of the width when the lamellar ex- 

 tensions of the shell are removed, the hinge-line less than one-half the 

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

 near the mid-length of the shell. The dimensions of two complete indi- 

 viduals of about average size are : length 21.5 mm. and 21 mm., width 

 25 mm. and 23.5 mm., thickness 13 mm. and 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 margins, and gently to the lateral and anterior margins; 

 the beak small and contiguous with that of the opposite valve, pierced by 

 a subcircular foramen; cardinal area narrow and inconspicuous, its 

 lateral margins more or less well defined ; the delthyrium broadly triangu- 



