ORTMANN: monograph of the naiades of PENNSYLVANIA. 33 



but particulars about the distribution of this form arc scarce. As far as can be 

 judged, A. peruviana seems to prefer \hv largest rivers, and j^robably muddj- 

 bottoms. In the Ohio from the Pennsylvania state line down to Portsmouth, 

 Ohio, I have never seen a true peruviana. 



In the distribution of the forms of the A. plicata-gronp two facts should be 

 especially emphasized. 



1. We have here a grouj) of at least three forms: a dwarfed form from Lake 

 Erie (A. plicata) ; a flat form found in the smaller rivers and headwaters {A. plicata 

 costata); and a swollen form with high beaks in the largest rivers {A. peruviana). 

 Only the first two are found in Pennsylvania, but the third turns up in the Ohio 

 in the neighborhood of Cincinnati. These conditions correspond in a degree to 

 what we have observed in two cases in the genus Fusconaia (See above, pp. 14 and 24) . 



2. In western New York, we have the typical form from small rivers, (costata) 

 tributaries of the St. Lawrence system, apparently entirely isolated from the 

 rest of the range, for this form is positively absent in the uppermost Allegheny- 

 drainage. The form found in Lake Erie is not this, but A. plicata. Thus it seems 

 that western New York has been colonized from the lake, the lake-form (plicata) 

 again assuming the river form (costata). This case should be compared with 

 what we have learned about Fusconaia flava (See p. 25). 



Genus Quadrula Rafiinesque (1820). 

 Ortmann, 1912, p. 250; Simpson, 1914, p. 811. 



Type Obliquaria metanevra Rafinesque. 

 Five well-defined species and one variety are found in Pennsylvania. 



Key to the Species and Varieties of Quadrula. 

 Oj. Shell more or less regularly rounded, with tubercles, which arc rather small and irregularly scattered, 

 and have no connection with the weakly developed, concentric beak-sculpture. Epidermis 

 yellow to brown, rayed, when young, rays green, often very broad and broken up into large 

 spots. 



&i. Shell rounded, high, not transverse. Posterior wing not developed Q. pustulosa. 



62. Shell more transverse. Posterior wing better developed (,>. pustulosa schoolcraftensis. 



ao. Shell not rounded, but trapezoidal, triangular, or elongated. Sculpture consisting of tubercles, 

 knobs, or ribs, of a more or less definite arrangement, more or less related to the beak-sculpture; 

 the latter double looped or of the zig-zag type. Epidermis rayed in various patterns or spotted. 

 hi. Shell subtrapezoidal, with a narrow and blunt posterior ridge. In front of the latter a broad 

 and shallow groove. Tubercles of posterior ridge not very large. Epidermis with ir- 

 regular rays, rays often spread out, but not spotted. 

 Ci. Shell about as long as high. Groove of disk distinct, generally without nodules. 



Q. quadrula. 



