268 AMERICAN JOURNAL 



without radiating lines ; umbonal slope raised, rounded with close, 

 crenulated, radiating lines, extending to the posterior margin 

 and disposed to bifurcate towards the base ; beaks nearly term- 

 inal. 



Locality. — Yorktown, Va. 



ASTARTIDiE. 

 ERYCINELLA, Conrad. 

 E. ovALis, Conrad.— PI. 22, fig. 2. 

 Miocene, Fossils, p. 42, pi. 42, fig. 5 

 Locality. — Yorktown, Va. 

 The figure represents the shell of the natural size. 



MACTRID^. 



MACTRA, Lin. 



I have followed H. and A. Adams in selecting the first species 

 Linnaeus described under the head of 3Iactra — 3L spengleri — 

 which was made the type of a new genus by Dr. J. E. Gray, to 

 which he gave the name of iScissodesma. Where a group of 

 genera are described as one genus, the diagnosis must necessarily 

 be general, and therefore Linnpeus did not give attention to minute 

 characters which may nevertheless indicate some important ana- 

 tomical difference. If we reject the first described species as 

 the type of a genus we must be guided by fancy alone in mak- 

 ing another species of the group the generic type. In this case, 

 as few authors would agree upon the selection, inevitable confu- 

 sion must follow. In 3£ spengleri there is an oblique triangular 

 groove opening into the upper edge of the cartilage pit, which is 

 made an essential character in distinijuishinor 3Iactra from Tri- 

 gonella., Mulinea, &c. In geographical distribution this subdivi- 

 sion of Linnsean genera presents remarkable and important facts. 

 Thus only one species of true 3Iactra has been found in the 

 waters of the Western Hemisphere — the 3L Cruadeloupensis, 

 Recluz — whilst the others inhabit China and Africa and the 

 eastern seas. 31. tumida, Reeve, inhabiting St. Thomas, is not 

 the tumida of Chemnitz, which inhabits China. As I have not 

 seen the former shell I do not know that it is a 3Iactra. 



M. CONTRACTA. — PL 22, fig. 6. 



Description. — Triangular, inequilateral, subcompressed, con- 

 tracted from beak to base ; anterior extremity acutely rounded; 

 umbonal slope carinated ; posterior hinge margin oblique, nearly 

 straight, obliquely subtruncated above the extremity. 



Locality. — N. Carolina ? 



