26 UNITED STATES GEOLOGICAL SUKVEY OF THE TkEElTORIES. 



wider, rounded furrows, in which fine, oblique, radiating, and concentric striae 

 may sometimes be seen; the latter of which, on well-preserved specimens, 

 also cross the costae, imparting a subcrenatc appearance to their crests. 



Length, 0.54 inch; breadth, 0.67 inch; convexity, about 0.16 inch. 



I have only seen the oblique radiating strife mentioned above between 

 some of the lateral costae of one specimen. In this they are arranged as if 

 they curve laterally, as they radiate from the direction of the beak. They 

 are very fine, regular, and appear to be generally obsolete on the specimens 

 examined. The fine concentric striae are smaller, and also generally obsolete. 

 In crossing the costae, some of the latter, at very regular intervals, seem to 

 be raised in a somewhat imbricating manner, so as to give the crests of the 

 same a crenate appearance where not worn. In the general, however, these 

 finer markings are nearly or quite obsolete. 



In size and general appearance, this little shell resembles Peden perplan us 

 and P. venustus, Morton, but has larger costae and differently-formed ears from 

 the latter. Its costae agree better in size with those of P. perplanus; but if 

 the ears of Dr. Morton's figured specimen were unbroken, they have a very 

 different form; and, besides, his species is said to be from the Eocene, 

 although supposed by him to be from- the Cretaceous. 



Locality and position. — Yellow Stone River, 150 miles from its mouth; 

 in a bed containing fossils of the Fort Pierre and Fox Hills groups of the 

 Upper Missouri Cretaceous. 



Genus SYNCYCLONEMA, Meek. 



Synon.—Pecten (sp.), Hall and Meek (1856), Mem. Am. Acad. Arts and Sei., V (u. s.), 331.— Meek and Hay- 

 deu (WOO), Proceed. Acad. Nat. Sci. Philad., 429 (not Miiller, 1776). 

 Syncyclonema, Meek (1864), Smithsonian Check-List N. Am. Cret. Fossils, 7. 



Eti/m.—a'vv, together ; KivtAof, a circle ; vnfia, a thread ; in allusion to its parallel concentric strise. 

 Type. — Pecten rigicta, II. & M. 



Shell small, compressed, nearly equivalve, vertically ovate, the height 

 being greater than the transverse diameter; hinge-line very short; ears very 

 small, the anterior being larger than the other; margins closed all around ; 

 no defined byssal sinus in either valve ; surface only showing concentric striae, 

 and sometimes stronger, regularly defined concentric ridges on the right 

 valve. 



The foregoing diagnosis is made out from the typical species only, the 

 exact limits of the genus being therefore unknown. It is possible that it inn) 



