128 niK NAL rii-ts. 



A NEW SPECIES OF PISIDIUM. 



BY V. STERKI. 



Pis. atlanticum, n. sp. 



Mussel of medium size, somewliat iiirtiiiipartite, oblique, short, aii- 

 jrular, rather well inflated : superior margin, moderately to ratlu^r 

 strongly curved, with the angles at tiie scutum and scutellum pro- 

 jecting and more or lessi-ounded ; supero-anterior slo[)e well marked, 

 steep, slightly curved to almost straight, anterior end a rounded 

 angle situated low; posterior [)art high, posterior margin (squarely) 

 truncate to subtruncate to rounded, int'ei'ior well curved; beaks some- 

 what nearer the posterior end, rounded, prominent over the hinge 

 margin, rather variable; surface dull to scmiewhat shining, with 

 striae very fine and crowded over the beaks and somewhat coarser 

 towards the margins; color straw or whitish to pale horn to grayish 

 or brownish ; shell opaque to more or less translucent, rather thin, 

 nacre glassy to whitish ; inner surface densely rugulose within the 

 pallial line, muscle insertions distinct ; hinge rather slight, curved, 

 plate narrow ; cardinal teeth : the i-ight curved to angular, its pos- 

 terior part thicker and grooved, the left anterior short, angular, the 

 posterior oblique, short, slightly curved, thin, less projecting than the 

 anterior; lateral teeth rather slight, the inner cusps of the right 

 valve not pointed, the outer quite small, those of the left valve 

 pointed, not very abrupt ; ligament slight. 



Size: long. 4.3, alt. 3.8, diam. 2.6 mill. 



Size: long. 4.5, alt. 3.9, diam. 3.1 mill. 



Habitat : New York to Florida and Mexico: Cedarville, N. Y., 

 eent by Mr. Walter Webb (No. 4853*), Lynchburg, Va., sent by 

 Mr. J. B. Henderson, from Sanderson Smith (No. 654), Lake Jessup, 

 Fla., sent by Mr. Bryant Walker (No. 3002); pools near We- 

 tumpka, Ala., sent by the same, collected by Mr. Smith in 1904 

 (No. 4963), De Kalb, Bowie Co., Tex., collected and sent by Mr. 

 Jas. H. Ferriss (No. 466), Texolo, Vera Cruz, Mexico, sent by Dr. 

 H. A. Pilsbry (No. 4746). From most places there were good 

 numbers of specimens, young to full-grown. Although evidently all 

 of one species, the specimens show considerable variation in regard 



*0f mv collection of Cvcladida;. 



