142 CAKBONIFEKOU8 I'KKIOI). 



AtJiyris svbtilita Newberry, 1S61, Exp. Exped. Colorado River, Paleontology, 126. 

 Aihyris suhiiUta Davidson, 18G3, Fossils of Southern India, pi. ix, tig. 7. 

 Afhyrin subfilita Meek, 1872, U. S. Geol. Surv. Nebraska, 180. 

 Compare AtJujris suhquadrata Hall, 1858, Geology of Iowa, i, i)art ii, 703. 



Shell of moderate size, varying considerably in outline, but is generally 

 subovate ; seldom, if ever, as wide as it is long, moderately gibbous, but 

 sometimes old shells ai-e much inflated. Ventral valve generally a little 

 more capacious than the dorsal : beak prominent, strongly incurved ; mesial 

 sinus not very deep, even at the front, and becoming obsolete about the 

 middle ; a more or less distinctly-impressed line usually exists along the 

 bottom of the sinus, extending from front to beak. 



Dorsal valve somewhat uniformly convex, but most prominently so 

 near the umbo ; beak small, slightly prominent ; mesial fold not distinctly 

 defined. 



Surface marked by concentric strise and by occasional imbricating 

 lines of growth ; faint traces of radiating strife, such as are conmion on 

 shells of this genus, are also occasionally seen. 



Length of a specimen of ordinary size, twenty-four millimeters ; 

 breadth, twenty millimeters ; height, nine millimeters. 



In all the variations this shell is subject to, it is easily recognized after 

 an acquaintance with the species has once been formed. One of the most 

 noticeable of its constant characteristics is the impressed mesial line at the 

 bottom of the sinus of the ventral valve, and extending from front to beak. 

 This feature is rarely obscure, and usually distinct. 



This species ranges through the whole series of strata of the Carbon- 

 iferous period into the Permian, according' to Mr. Meek ; but it has not yet 

 been recognized in the Siibcarboniferous rocks of America, unless S. suh- 

 lameUosa Hall, from the Chester limestone of Illinois, should prove to be a 

 variety of this sjiecies. In England and India, however, it has been recog- 

 nized in Subcarboniferous strata. Besides the localities at which it was 

 collected by the exploring parties, it has been obtained from the Carbon- 

 iferous strata of West Virginia, Ohio, Illinois, Missouri, Iowa, Nebraska, 

 and Kansas, and also from the Permian rocks of the latter State. 



Position and locality. — The follomng are the localities that have fur- 

 nished tlie species to the collections : — Carizo Creek, Maricopa County ; 



