1903 ] NATURAL SCIENCES OF PHILADELPHIA. 785 



6'.— Beaks nearly smooth; alt. about three-quarters and diam. 

 about one-half the length of the shell ; basal and hinge 

 margins about equally arcuate; finely striate, 

 c— Subequilateral, the two ends almost equally rounded; 

 olivaceous-gray, with chestnut beaks and usually yel- 

 low basal margin, S. jalayensis. 



c'__Inequilateral, the posterior end noticeably wider; yellow, 

 becoming fleshy-gray at the beaks; thinner, 



S. martensi. 



Sphaerium triangulare (Say). PI. LIII, figs. 4, 4a. 



Figures are here given of one of the two type specimens of this species 

 (PI. LIII, figs. 4, 4a). It measures, length 13, alt. 10.3 mm., agree- 

 ing with Say's measurements. ^^ The sculpture of concentric ridges or 

 coarse striiB is strong and a little irregular. The beaks are strongly, 

 regularly sculptured, as in S. striatinum. This is a very characteristic 

 feature of the species. The beaks arc nearly median; the dorsal mar- 

 gin of the valve is much more arcuate than the ventral, and the anterior 

 end tapers somewhat, the posterior being broadly rounded. The 

 anterior and posterior lateral teeth are of about equal length, single in 

 the left, double in the right valve. The anterior pair is decidedly 

 heavier than the posterior, the latter being finely crenulated. 



The types are both dead, and have lost the cuticle, but are otherwise 

 well preserved. They are probably from one of the lakes near the City 

 of Mexico, as there is a tray of specimens in the collection exactly like 

 them and in the same condition, from Lake Texcoco. 



In Lake Patzcuaro, Michoacan, the shells are smaller but somewhat 

 thicker (PI. LIII, figs. 3, 3a; PI. LIV, fig. 2). The cuticle is very 

 glossy, bright greenish-yellow with some gray streaks, or gray-brown • 

 with or without a yellow zone below. Sculpture as already described^ 

 but in these fresh shells some indistinct radii are visible. The beaks 

 are large and full. The ligament, though largely immersed, is conspicu- 

 ous externally. There is a narrow, slightly sunken escutcheon and a 

 wide, short, lunule bounded by faintly impressed lines. The teeth 

 (PI. LII, fig. 3, right valve) of this form are a trifle heavier than in 

 typical S. triangulare, but otherwise similar. On the right valve the 

 lower laterals seem to be borne on ridges running out from under the 

 hinge-plate. They are very short, high and triangular. Length 11.5, 

 alt. 9, diam, 7 mm. 



1" Prime expressed doubt about the specific identity of the two specimens 

 given to the Academy by Mrs. Say after Say's death, solely because the shell is 

 not especially triangular; but since these specimens agree completely with the 

 original description and measurements, and were labeled by Say, there seems 

 to be no good reason for doubting their authenticity. 



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