ORTMANN: UNIONID© OF WESTERN PENNSYLVANIA. 210 
south. The distribution of the two Avxodgonzas also should be investi- 
gated more closely. <Alasmidonta hetorodon appears to be restricted 
to the southeastern corner of the state. MMargaritana margaritifera 
should be looked for in the northeastern and the northern central parts 
of the state. The establishment of the southern boundary of distribu- 
tion of the latter species is very desirable. Unio fisherianus is a 
southern form, and may be present in the Potomac drainage in south- 
ern Pennsylvania. Unio productus Conrad, which comes very near 
to U. fisherianus, is known from the northern tributaries of the 
Potomac in Maryland (Pilsbry). The occurrence of U. fisherianus 
in White Clay Creek and in the Schuylkill probably marks the northern 
extremity of its range. From the Susquehanna it is unknown. 
Thus we see, that many questions still await final decision, and 
that much remains to be done in a section of the country, where 
collectors have been busy for almost a century. Before we know the 
actual distribution of each species, we cannot venture to say anything 
about their history ; but this should be the aim of all studies in geo- 
graphical distribution. 
