CARBO N I I KROUS AGE. PE RM I A N PERIOD. 53 



PTERIIX^E. (Sec page 28.) 



(Jonns ITMICROTIS, MI:I:K. 



n. GrypkiUi (ap.)i SCHLOT. Aca.l. Munch. 1816, 30; ib. Petref. 1820, 292. 



I .,,l,i (sp.), > " C. SOWEBBT, Tram. Oeol. Soc. Load. 2d cr. HI, 1829, 119, and of various other* (not 



KI.KIX ; LAHK.). 



.)/..,,.//., Kix.i, Catalogue Perm. Kons. 1849, p. 9 ; Id. Monogr. Perm. FOM. Great Brit. 1850, p. 164 

 MKKK & HATDKH, Trans. Albany Inst. IV, Marcli 2d, 1858. SWALLOW, Trans. St. Lonii Aoad. Sol. I, 

 1858. SIICXAKII, ib. 1859 (not RKOXH, 1830). 



Eumicrotit, MEEK, American Jonr. Sci. 2d ser. XXXVII, March, 1864, p. 216. 

 Ktyat.- '"V'f- small; T<, ear. 



if. .l/oiio/i'i Uavni, MEEK & HATPKN. 



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

 tin- ri^'ht flat, or <\cn a little concave; not distinctly auriculato, the ears being 

 nearly obsolete. JJeaks SH!M cntral, very slightly oblique, unequal, that of the left 

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

 projecting above the hin^e line. Hinge short, narrow, edentulous; cartilage 

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

 and separated from the hinge by a very small rudimentary car, which docs not 

 projeet l>< \ond 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 costac, much more distinctly 

 marked on the left than the right valve. 



The shells embraced in this genus arc 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. AH the 

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

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

 tute of any traces of radiating costac or stria? ; while they are all marked with more 

 or less distinct and regular concentric costae or undulations, as in Inoceramna. 

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

 I'.itin'n /<>/ /x. 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 Eumicrotw. In 

 addition to these differences, Count Keyserling's figures (Peischora 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. spcluinur'ni, 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 Monotin .-<t/iii<in'<t, the type of Bronn's genus. This shell, it will 



