ORTMANN: monograph of the naiades of PENNSYLVANIA. 79 



L. H. D. Pr.ct. 



Size: 1. Mt. Morris, Cat. No. 61.4574 Ill mm. 8S mm. 4.3 mm. .39 



2. Larabee, Cat. No. 61.3350 100 " 81 " 49 " .49 



3. Eastbrook, Cat. No. G1.3349 88 " 67 " 32 " .36 



4. Mahoningtown, Cat. No. 61.3907 69 " 59 " 32 " .46 



Soft parts (See Ortmann, 1912, p. 263) : I have seen several hundred females, 

 many of them gravid, but in no case I have found marsupial structure in the inner 

 gills. Glochidia (See Ortmann (19116, PI. 89, fig. 4) measurements: 0.15 X 0.15 

 mm., and Surber (1915, p. 7, fig. 9) measurements: 0.16 X 0.16 mm.). 



Breeding season: Gravid females have been found on May 13, 1911; May 23, 

 1909; May 23, 1911; May 23, 1912; May 24, 1911; June 18, 1908; June 22, 

 1908; June 23, 1910; July 4, 1909; July 8, 1909; July 19, 1909. Glochidia have 

 been found on Jime 18, June 23, and July 4. Thus this form is tachytictic, breed- 

 ing from the middle of May to the middle of July, and beginning to discharge the 

 glochidia in the second half of Jime. 



Remarks: This form has been hitherto regarded as a good species. In its 

 typical environment (small streams) it is, indeed, quite constant, although it 

 varies a good deal. The shape maj' be different, from subrotund to subovate, or 

 subtriangular, and quite often the lower posterior end is drawn out and even 

 deflected, thus producing an outline recalling that of P. obliquum rubrum. Gen- 

 erally, there is no trace of a radial furrow, and the lateral faces are rather flat, 

 only slightly convex, and without a posterior ridge. In obesity, this form varies 

 greatly, and more swollen shells (See No. 2, under measurements) gradually pass 

 into P. obliquum catillus, and this takes place in the downstream direction. In the 

 upper Allegheny occasional individuals turn up, having the dimensions of catillus; 

 such become more abundant in Armstrong Co., where they outnumber the coc- 

 cineum-ioYms. Here also individuals turn up, which show the beginning of a 

 radial furrow, which then lead to specimens with well-developed furrow, represent- 

 ing typical obliquum. 



There is complete intergradation between these forms, and it is impossible 

 to separate them except by drawing an artificial line. 



Below Pittsburgh, in the Ohio, only a single coccineum has been found. Con- 

 ditions similar to those in the Allegheny prevail in the Beaver-drainage. 



The nacre of coccineum is of a reddish tint more frequently than is the case 

 in any of the preceding forms. 



Localities in Pennsylvania, represented in the Carnegie Museum: 



Ohio proper and Beaver-drainage: 

 Ohio River, Cooks Ferry, Beaver Co. 



