CARBONIFEROUS AGE. PERMIAN PERIOD. 53 



Subfamily PTERIIN^. (See page 28.) 

 Genus EUMICROTIS, Meek. 



Synon.—GrypUtes (sp.), Sohlot. Aoad. Muncli. 181G, 30 ; ib. Petref. 1820, 292. 



Avicula (sp.), J. DE C. SowEEBY, Trans. Geol. Soo. Loud. 2d ser. Ill, 1829, 119, and of various others (not 

 Klein; Lamk.). 



Monotis, King, Catalogue Perm. Foss. 1849, p. 9 ; id. Monogr. Perm. Foss. Great Brit. 1850, p. 154. 



Meek & Hayden, Trans. Albany Inst. IV, March 2d, 1858. — Swallow, Trans. St. Louis Acad. Soi. I, 

 1858.— Shomakd, ib. 1859 (not Bronn, 1830). 

 Eumicrotis, Meek, American Jour. Soi. 2d ser. XXXVII, March, 1864, p. 216. 

 Eiym. — eu, very ; /uinpo'?! small ; cJc, ear. 

 Type. — Monotis Hawni, Meek & Hayden. 



Shell suborbicular, plano-convex, the left valve being usually very convex, and 

 the right flat, or even a little concave ; not distinctly auriculate, the ears being 

 nearly obsolete. Beaks sub-central, very slightly oblique, unequal, that of the left 

 valve often elevated, gibbous and incurved; the other very small, and scarcely 

 projecting above the hinge line. Hinge short, narrow,,^ edentulous ; cartUage 

 cavity under the beaks (King). Byssal notch or sinus of right valve narrow, deep, 

 and separated from the hinge by a very small rudimentary ear, which does not 

 project beyond the margin. Adductor muscular scar large and sub-central, im- 

 pressions of retractor muscles several, small and placed near the beaks. Surface 

 generally with radiating, more or less vaulted or scaly costse, much more distinctly 

 marked on the left than the right valve. 



The shells embraced in this genus are apparently most nearly allied to Aucella 

 of Keyserling, to which Prof McCoy refers them. Although Count Keyserling's 

 genus has not been generally adopted, it seems to be entirely distinct from all the 

 allied groups, and has been clearly defined by its distinguished author. AU the 

 species upon which it was founded, however, differ from those of the group under 

 consideration, in being much more obhque, more oval in form, and entirely desti- 

 tute of any traces of radiating costee or striae ; while they are all marked with more 

 or less distinct and regular concentric costse or undulations, as in Inoceramus. 

 Again, they have the right or smaller valve proportionally more ventricose than in 

 Eumici'otis, and also possess a minute, internally concave, sharply defined anterior 

 ear under the beak of the left valve, never seen in the group we are describing. 

 Another difference is the entire absence of the lobed appearance of the posterior 

 side of the valves in Aucella, so often seen in the typical forms of Eumicrotis. In 

 addition to these differences. Count Keyserling's figures (PetscJiora Land, tab. 16) 

 show that in the type of his genus the scar of the adductor muscle is nearly 

 marginal; and that there is no distinct cartilage cavity under the beaks; while 

 according to Prof. King, there is in E. speluncaria, Schlot. (sp.). 



That the group of shells we are describing are not congeneric with Monotis of 

 Bronn, must be manifest to any one who will take the trouble to compare one of 

 these forms with Monotis salinaria, the type of Bronn's genus. This shell, it will 



