ATHYRIS 465 



the crura. The spiral cones of the brachidium lie base to base with the 

 apices directed laterally, the crura are long and convergent with the pri- 

 mary lamelln?, abruptly recurving at their points of origin, the jugum is 

 saddle shaped in its main portion and is situated posterior to the middle 

 of the shell, the anterior extremity of the saddle may be simple or divided, 

 posteriorly it is narrowed and inclined towards the beak of the valve for 

 a short distance, thence rises abruptly towards the umbo of the pedicle 

 valve, bifurcates near the extremities of the crura, each branch following 

 the curvature of the primary lamella, so forming accessory lamellae which 

 lie between the primary lamella? and the first band of the secondary 

 lamellfe. The surface of the valves is marked by broad, lamellar expan- 

 sions of the shell at each growth line. 



Remarks. All the atheroid shells are characterized by a hinge-plate 

 with an apical perforation, and in the internal casts of the shells, the fill- 

 ing of the cavity beneath the hinge-plate, running up as a finger-like 

 point to the perforation and directed towards the beak of the opposite 

 valve, is a very conspicuous feature of the brachial valve. The several 

 generic groups of these species are differentiated largely by the general 

 outline of the shell, and especially by the nature of the surface markings. 

 In Athyris proper the lamellar extensions of the shell are highly charac- 

 teristic. 



ATHYRIS LAMELLOSA (Leveille) 

 Plate LXXVIII, Figs. 1-5 ; 15-20 



1835. Spirifer lamellosus Leveille, Mem. Soc. Geol. de France, 1st ser., 



vol. 2, p. 39, figs. 21-23. 

 1875. Athyris lamellosa f Meek, Pal. Ohio, vol. 2, p. 283, pi. 14, figs 



6a-b. 

 1888. Athyris lamellosa Herrick, Bull. Sci. Lab. Denison Univ., vol. 3, p. 



49, pi. 2, fig. 7. 

 1888. Athyris ashlandensis Herrick, Bull. Sci. Lab. Denison Univ., vol. 4, 



p. 24, pi. 3. fig. 6. 



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



fig. 10. 



1895. Athyris ashlandensis Herrick, Geol. Surv. Ohio, vol. 7, pi. 23, fig. 



10. 

 1895. Athyris lamellosa Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, pi. 46, 



figs. 16-20. 

 1899. Athyris lamellosa "Weller, Trans. St. Louis Acad. Sci., vol. 9, p. 21, 



pi. 4, fig. 8. 

 1899. Athyris lamellosa Girty, Mon. U. S. Geol. Survey, vol. 32, p. 561, 



pi. 71, fig. 7a. 

 1904. Athyris lamellosa Girty, Prof. Paper U. S. Geol. Survey, No. 21, 



p. 49, pi. 10, figs. 12-13. 

 1909. Athyris lamellosa Weller, Bull. Geol. Soc. Am., vol. 20, p. 312, pi. 



14, figs. 5-6. 



