1896.] NATURAL SCIENCES OF PHILADELPHIA. 565 



at end of hinge line, and connected therewith by a short impression ; 

 prismatic layer at margins of valves narrow and bluish-green. 



Length 14, alt. 7'8, diam. 5*4 cm. 



Length 14*5, alt. 8'1, diam. 5"5 cm. 



Kio de la Plata. Described from seven specimens in the collection 

 of the Academy of Natural Sciences of Philadelphia. 



This species is named in honor of Mr. Charles Torrey Simpson, 

 whose valuable papers upon the Unionidce have been of great ser- 

 vice to students of this intricate and difficult group. 



G. Simpsonianus belongs to the group of G. trapesialis Lam. It 

 differs from typical trapesialis (Encycl. Meth., pi. 205) in being oval 

 rather than subtriangular ; the beaks are far less inflated, low and 

 wide ; the nacre is peculiarly pearly, having the luster of that of 

 the pearl oyster ; the hinge line is more nearly parallel with the 

 basal margin and is far longer in proportion to the length of the 

 shell ; the posterior large muscle-scar is close to the sinus at end of 

 hinge-line, not distant from it as in trapesialis ; the foot protractor 

 scar is of a very different shape. Finally, the shell, while smaller, 

 is much more ponderous and thick than trapesialis. Well-grown 

 specimens of trapesialis measure 19 cm. long, and are thinner than 

 Simpsonianus 14 cm. in length. 



Anodon penicillatus Gray* apparently resembles this species in 

 the internal markings (which are common to many species of Gla- 

 haris), but it is described as "Antice suhcompressa, rotundata, sub- 

 gracili," terms applying well to some forms of G. trapesialis var. 

 exoticus. 



The great solidity of the shell for a Glabaris will separate the 

 species from G. trapesialis var. exoticus Lam. and var. riogranden- 

 sis V. Iher. It resembles G. Forbesianus Lea in the thickness of the 

 shell, but is more oblong, with longer hinge-line, wider beaks, 

 differently shaped protractor pedis scar, and wider ventral gape. 



*Proc. Zool. Soc. Lond., 1834, p. 57. 



