The Nautilus. 



Vol. XXIX. OCTOBER, 1915. No. 6 



NOTES ON AMERICAN SPECIES OF MACTRELLA. 



BY WILLIAM H. DALL. 



The species of MactreUa are extremely elegant shells, especially 

 those with concentric undulations. There are but few of them 

 altogether, and usually not more than two species in any given 

 fauna, of which one is usually smooth and the other undulated, 

 as in the case of the genus Labiosa. They are so fragile that 

 they are generally broken by the waves when cast on the beach, 

 and, being burrowers, are rarely obtained by the dredge, hence 

 are rare in collections. They are confined to the warmer waters 

 of the globe and perhaps most numerous, though not of largest 

 size, among the islands off the coast of southern Asia. 



There is but one species in the American Atlantic tropics, 

 the Mactra alata Spengler, 1802. Gmelin confounded it with 

 the M. striatula Linn6,and Lamarck gave to the same combina- 

 tion of the hinge elements, Gray in January, 1853, founded his 

 genus MactreUa^ while in April of the same year Morch proposed 

 for it the genus Papyrina. 



The only other East American species known to date is the 

 MactreUa iheringi Dall (Nautilus, Mar. 1897 ; figured in Proc. 

 U. S. Nat. Museum, 1902, pi. 32, fig. 8) from the const of 

 southern Brazil, which approaches more nearly to the typical 

 Mactras in general form than any other species of the genus. 



The West American analogue of M. alata is the M. exoleta 

 Gray (1838) which differs by having the margin of the escut- 



