INVERTEBRATE TAL^ONTOLOGY. 223 



adopted is that, it commenced its existence during the Jurassic period, and 

 continued until the present time. Dr. Stoliczka, however, doubts its exist- 

 ence j)i"evious 1o the Tertiary, and thinks the Jurassic and Cretaceous species 

 usually referred to it belong to Gorymya. 



It is quite possible, as we have elsewhere suggested long back, that the 

 species here described may more properly belong to the genus Corymya than 

 to Thracia ; especially as the cardinal margin behind the beaks, as nearly as 

 can be determined from casts and impressions, seems hardly to have been so 

 strongly inflected in these shells as in the latter genus. The objection, how- 

 ever, to referring them to Corymya, is, that two of them show thepallial line, 

 with its rather deep sinus, to be well defined, as in Thracia; while in all the 

 numerous species of Corymya described by Professor Agassiz and others, not 

 a trace of the pallial line has ever been seen. 



The genus Thracia is certainly known to have been represented during 

 the Tertiary epoch ; and some eighteen or more species are known to inhabit 

 our existing seas. The living species are widely distributed in the American, 

 Chinese, and British seas, as well as in the Mediterranean and on the coast 

 of Greenland, Norway, &c. 



Thracia! snbtort uosa, M. & H 



Plate 37, fig. 5. 



T> Vina suhtorluosa, Meek and Hayden (1856), Proceed. Acad. Nat. Sci. Pbilad., VIII, 272. 

 Thracia subtortuosa, Meek (1864), Smithsonian Check-List N. Am. Inv. Cret. Foss., 14. 



Shell transversely elliptical, compressed, and a little warped ; anterior 

 side rather narrowly rounded ; posterior side subtruncate at the extremity, 

 and provided with an obscure ridge, which (on casts) extends from the back 

 part of the beaks obliquely backward and downward in the direction of the 

 postero-basal border; pallial margin nearly straight, or forming a broad 

 gentle curve, a little more prominent before than behind the middle, more or 

 less arcuate laterally ; dorsal margin declining slightly, with a convex outline 

 in front of the beaks, and nearly horizontal behind them ; beaks small, 

 depressed, not very distinct from the dorsal border, located somewhat in 

 advance of the middle; surface unknown. 



Length, 2.23 inches ; height, 1.27 inches ; convexity (of a left valve), 

 about 0.20 inch. 



The only specimen of this species yet known is an internal cast of a left 



