78 MEMOIRS OF THE CARNEGIE MUSEUM. 



into coccineum. It is interesting to see that it actually exists in the Mahoning, 

 for one of the original localities mentioned by Lea for his U. solidus, is "Mahoning 

 River, Ohio." 



In the lower Allegheny and in the Ohio just below Pittsburgh, tliis form is 

 even more abundant than the normal P. ohliquum. It is found under the same 

 conditions as the latter, but most abundantly upon gravel bars with strong current. 



General distribution: Type locality, Scioto River, Ohio (Conrad). 



This variety seems to be found wherever the typical form is present, that is to 

 say, in the Ohio-, Cumberland-, and Tennessee-drainages; but particulars cannot 

 be accurately expressed on account of the confusion which prevails with regard 

 to it. It is certainl.y present in the upper Tennessee-drainage, going upstream 

 about as far as P. ohliquum. In addition this variety seems to be rather frequent 

 west of the Mississippi, in regions where the typical P. ohliquum is largely absent 

 or doubtful, namely in Iowa, Missouri, and northern Arkansas. As has been 

 indicated above, P. catillus seems there to replace P. ohliquum, and in turn connects 

 with the var. ruhrum. This group of forms {catillus-ruhrum) develops farther to 

 the southwest , into a peculiar assemblage, which cannot be discussed here. In 

 Missouri there is shown a tendency on the part of catillus to pass into coccineum 

 in small streams, exactly as is the case in Pennsylvania. 



Pleurobema obliquum coccineum (Conrad) (1836). 

 Quadrula coccinea (Conrad) Simpson, 1914, p. 883. 



Plate VII, figs. 3, 4, 5. 

 Records from Pennsylvania: ; 



Conrad, 1836 (Mahoning River, near Pittsburgh)." 

 Marshall, 1895 (Allegheny River, Warren Co.). 

 Rhoads, 1899 (Beaver River, Wampum, Lawrence Co.). 

 Ortmann, 19096, p. 200. 



Characters of variety: Differs from P. ohliquum by the flat and compressed 

 shell, less prominent beaks, and the absence of the radial furrow. In the latter 

 character, it agrees with P. ohliquum catillus, but differs from this in the com- 

 pression, the diameter being less than fifty percent of the length. The shape of 

 the shell is subtriangular or, on account of the low beaks, more or less subovate. 

 Color of nacre very variable: white, salmon, or all shades of pink to deep red. 



" The nearest point of the Mahoning to Pittsburgh is where it joins the Shenango to form the 

 Beaver, at Mahoningtown, LawTence Co., Pa. I have found this form at this place, and this should be 

 regarded as the Type locality. 



