19t) UNITED STATES GEOLOGICAL SURVEY OF TUB TERRITORIES. 



Specimens of this genus have generally been found in the condition of 

 internal casts, so that its hinge-characters seem not to have been made out 

 with entire clearness ; consequently, scarcely any two authors agree exactly 

 in regard to the details of its hinge-teeth. I have preferred, however, to. 

 follow Sowerby's original description of these parts. The most characteristic 

 and the most readily-determined peculiarity of the genus, however, is its 

 remarkably deep, angular, pallial sinus, directed nearly vertically, and often 

 extending almost to the beaks. 



This genus has been confounded, by Woodward, H. and A. Adams, 

 and others, with Poromya, Forbes, an entirely distinct group, with altogether 

 different hinge-characters, and a merely slightly sinuous pallial line. 



I cannot agree with Dr. Stoliczka that there is any necessity for chang- 

 ing Sowerby's name Thetis, for this genus, to Thetironia, merely because 

 Linnaeus had previously used the name Tethys for another genus; the two 

 names Thetis and Tethys being distinct in orthography as well as in mean- 

 ing, It is true that Cuvier, and perhaps some others, had written the name 

 of Linnaeus' s genus Thetys and Thetis; but this does not alter the case, 

 because these authors had no right to change Linnaeus's name in this way 

 from its original orthography. 



So far as known at this time, the genus Thetis seems to be entirely 

 confined to the Cretaceous rocks. 



Thetis? circularis, M. & H. 



Plate 17, tigs. 8, a, b, c. 



Vauis? circidaris, Meek and Hayden (1856), Proceed. Acad. Nat. Sci. Philad., VIII, 272. 

 Cyclinat circularis, Meek (1804), Suiithsouiann Check-List N. Am. Cret. Fossils, 13. 



rig. is. Fig. io. Shell subcircular, ventricose, very thin; basal, ante- 



rior, and posterior margins rounded ; postero-dorsal border 

 declining from the beaks, with a moderately convex outline ; 

 pallial margin smooth within ; beaks elevated, gibbous, 

 ^ThetM eireuians" pointed, and curved obliquely forward and inward, located 

 m. & h. slightly in advance of the middle; surface nearly smooth, 



Fig. 18. Shows hinge . . , .,, „ , , . , . •■! 



and interior of left, or only marked with fine, obscure, concentric striae, with 

 valve - some appearance of a few minute, irregularly scattering 



Fig. 19. Shows dorsal l ' . 



outline of same, punctures ; muscular impressions very faintly marked, 1 he 



and its projecting . , ', n • i „<.„ q 



hinge-teeth. anterior one narrow-subovate, and occupying an elevated, 



nearly marginal position ; posterior muscular scar broader than the other, 

 and placed rather low and near the anal margin. Pallial sinus funnel-shaped, 



