197 ANNALS OF THE CARNEGIE MUSEUM. 
Wright) ; while the eastern form should be called vavzcosa (Lamarck) 
(1819). Nevertheless I do not think that the two are specifically 
different. In our mountain streams forms are frequently encountered, 
which cannot be without doubt assigned to the western form, as in 
fact they closely resemble the eastern form in essential characters. 
Such individuals are also found in the Allegheny. The typical mar- 
ginata (western form) does not begin to prevail till we come to the 
northwestern section of the state. 
32. Unio gibbosus Barnes. 
Mentioned in Harn’s list as from western Pennsylvania; recorded 
by Stupakoff from Allegheny County, and by Rhoads from the Ohio 
and Beaver. 
Ubiquitous all over the Ohio drainage in the state, extending east- 
ward to Somerset, Indiana, and McKean Counties. In fact this spe- 
cies is so generally distributed, and so abundant, that there is hardly 
a stream which has mussels, in which it is missing. ‘The most remark- 
able instance of its absence is in the case of Raccoon Creek in Beaver 
County. Other cases, in which it has not been found, occur in those 
creeks, the fauna of which has been more or less extirpated, as for 
instance Connoquenessing Creek in Butler County. 
33. Unio crassidens Lamarck. 
It is reported by Harn from western Pennsylvania, by Clapp from 
Allegheny County, by Rhoads from the Ohio, and by Call from the 
Allegheny River. 
It is restricted to the larger rivers, and is abundant, where found. 
I have taken it in the Ohio in Beaver and Allegheny Counties ; in the 
Allegheny in Allegheny and Armstrong Counties. It used to be 
found in the Monongahela at Charleroi, Washington County (Ehr- 
mann Collection). It does not occur elsewhere. 
34. Pleurobema clava (Lamarck). 
Reported only by Call and Harn from western Pennsylvania. 
It avoids the large rivers, and has never been found in the Ohio, 
the Monongahela, and the Allegheny as far up as southern Armstrong 
County. In Armstrong County it is rare in the Allegheny. It is 
found in the Beaver drainage in Lawrence and Mercer Counties, in 
Little Beaver and Raccoon Creeks in Beaver County, in the Cheat 
River in Fayette County, in the Loyalhanna and (formerly) in the 
Conemaugh in Westmoreland County, in the Upper Allegheny in 
Venango and Forest Counties, and in French Creek in Venango and 
Crawford Counties. It is nowhere abundant; the largest number 
