ORTMANN: monograph of the naiades of PENNSYLVANIA. 83 



Pleurobema obliquum pauperculum (Simpson) (1900). 



Quadrula coccinea paupercula Simpson, 1900, p. 789; Quadrula subrotunda Sterki, 

 1907a, p. 391 (partim); Quadrula subrotunda Ortmann, 19095, p. 203 {per 

 errorem) ; Quadrula coccinea magnalacuslris Simpson, 1914, p. 884.^'^ 



Plate VII, fig. 6. 

 Records from Pennsylvania: 



Ortmann, 19096, p. 203 (as form of Q. subrotunda). 



Characters of variety: Resembling P. obliquum coccineum, but shell consider- 

 ably smaller, more convex (diameter ranges around fifty percent) , with the outline 

 subovate, and generally more elongate, and the growth-lines more distinct and 

 quite regular. The beaks are not very prominent (hence the subovate, and not 

 triangular, outline), and a furrow, except in extreme cases is absent, there being 

 merely a flattening in its place. Nacre always white. 



L. H. D. Pr.ct. 



iSwe; 1. Erie, Cat. No. 61.4393 (largest at hand) 71mm. 50 mm. 35 mm. .49 



2. Erie, Cat. No. 61.4393 70 " 55 " 34 " .49 



3. Erie, Cat. No. 61.4579 62 " 48 " 28 " .45 



4. Erie, Cat. No. 61.3930 (smallest at hand) .. .55 " 47 " 29 " .53 



Soft parts: Only two individuals have ever been found alive by mj'self, of 

 which the soft parts of one were preserved. It proved to be a male, with the 

 structure agreeing with that of P. obliquum. The characteristic features of the 

 females have not been observed. The color of the soft parts was grayish-white. 



Glochidia and breeding season unknown. 



Remarks: As Simpson correctly recognized, this is a form dei-ived from coc- 

 cineum, and not directly from obliquum: nevertheless, the rules of nomenclature 

 force us to use the above name. 



This is the Lake Erie form of coccineum, descended from the stock which 

 crossed the divide between the upper Ohio and the lake-drainages in northern 

 Indiana, Ohio, and possibly Pennsylvania. This lacustrine form has the distinct 

 and regular growth-lines, which are also more closely set, found so often in shells 

 from Lake Erie, and this peculiarity together with its small size are its chief diag- 

 nostic characters. In shape it is generally more elongate-ovate than coccineum, 

 but, according to the growth-lines, young specimens must be shorter and more 

 rounded-ovate. On the average it is slighth' more swollen than coccineum, but 



^ The change of the varietal name introduced by Simpson in 1914 is unnecessary. There is a 

 Unio pauperculus Lea (1861) but this does not constitute a pre-occupation of the term Qiiadrvla coccinea 

 ■paupercula. 



