562 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



PI. XIV, fig. 6), or Hall and Clarke (loc cit, PI. XLVI, figs. 16-20). This 

 will be seen by comparing the figure given on PL VI, fig. la, which repre- 

 sents a somewhat crushed, exfoliated specimen, with the illustration above 

 cited. The rather unusual variation in shape among these forms, especially 

 in regard to the length and straightness of the hinge line, may in part be 

 accounted for by the fact that some of the specimens are casts, "the thick 

 shell of the rostral region when present causing the hinge to appear shorter." 

 (Meek, loc. cit., p. 285.) 



I suspect that none of the forms will prove to be quite identical with 

 A. lamellosa of Leweille. Compared with Leveilld's illustrations the speci- 

 mens from the Yellowstone National Park are less elongated transversely, 

 and lack the high, sharp fold and sinus that characterize the type, which has 

 in addition the projecting lip of the anterior margin bilobate through an 

 acute emargination. A still further point of difference is that in the 

 American forms the beak is uniformly smaller and incurved so as to conceal 

 the foramen, which is well shown in the type. 



Athyris ashlandensis of Herri ck may be a synonym of A. lamellosa. 



In the exfoliated condition in which the shell occurs it might be referred a 

 priori to either Athyris (sensu stricto) or Cliothyris. The only related forms 

 with which it is necessary to compare it are Athyris incrassata and Cliothyris 

 crassicardinalis White, and CI. roissyi Walcott. It can without difficulty 

 be distinguished from CI. crassicardinalis, for it is much larger and more 

 transverse, with a distinct though low median fold and sinus. On the 

 other hand, it is considerably smaller than either A. incrassata or CI. roissyi 

 Walcott. The shape is transversely elliptical, with a long and nearly 

 straight hinge line, while the other forms are subquadrate in outline, with a 

 much curved hinge margin. 



Formation and locality: Madison limestone. Waverly to Keokuk, 

 Europe; Sciotoville, Ohio; Licking County, Ohio; Lebanon, Kentucky; 

 Crawfordsville, Indiana; New Mexico. 



Athyris incrassata Hall(?) 



Athyris incrassata Hall, 1858: Geol. Surv. Iowa, Vol. I, Pt. II, p. 600, PI. XII, fig. 6. 

 Hall and Clarke, 1893: Pal. New York, Vol. VIII, Pt. II, p. 90, PI. XLVI, fig. 

 21; PI. LXXXIII, fig. 39. 



The shell for which 1 have used this name is represented only by an 



imperfect cast of the ventral valve. It must have measured between 50 and 



