564 PROCEEDINGS OF THE ACADEMY OF [1896. 



curved, as in certain middle European forms of A. cygnea. The 

 specimens are also even thinner than examples of riograndends be- 

 fore me, of equal size. 



Pond and a small creek near Maldonado, Uruguay (Dr. Rush). 



The differences between this form and typical trapesialis are mani- 

 fest when we compare the typical figures of the latter in Encycl. 

 Meth., pi. 205, which agree perfectly with specimens before me. The 

 divergence between the several geographic races of G. trapesialis, 

 such as riograndensis, exoticus and cygneiformis render it necessary, 

 in my opinion, to recognize these as of subspecific rank. The ex- 

 treme " lumpers " do not seem to understand that if evolution of 

 species by divergence is granted, " subspecies " are a necessary con- 

 sequence, whether we distinguish them by name or not. Every 

 practical zoologist knows that they exist, and are neither more nor 

 less artificial or subjective conceptions than " species ; " and it 

 seems a truer method to recognize certain races in which more or 

 less definite characters are correlated with geographic range, than 

 to lose sight of the differences induced by causes acting over whole 

 districts or river-systems by lumping unlike forms under "species" 

 which are equally with subspecies, arbitrary groupings. 



Glabaris Simpsonianus n. sp. PI. XXVII, fig. 13. 



Shell oblong-oval, ventricose, very inequilateral, thick, solid and 

 heavy ; greatest diameter about in the middle ; basal margin gaping 

 from the anterior extremity nearly two-thirds the distance to poste- 

 rior end ; dorsal margin gaping slightly from the end of hinge to 

 the posterior end of shell ; externally green toward the beaks, the 

 greater part of the surface olivaceous, blackish brown at the ends 

 and basal margin, the posterior dorsal slopes biradiate with green ; 

 the surface smooth and polished, with rather coarse, low wrinkles of 

 growth, more crowded and somewhat lamellose at the ends and basal 

 margin. Upper and basal outlines about equally curved ; hinge 

 margin long, wide, somewhat sloping, gently curved, rounded or 

 hardly angular at the ends ; posterior margin sloping above, 

 rounded below ; anterior end somewhat narrower, rounded ; beaks 

 wide and low. 



Interior silvery or salmon-tinted, very pearly, usually showing 

 irregular black parallel lines in the neighborhood of the muscle im- 

 pressions and pallial line. Cavity of valves deep, of beaks shallow 

 and wide ; muscle-scars well impressed, the foot protractor scar un- 

 usually long ; posterior adductor scar situated very near to the sinus 



