42 Remarks on the Unios of the United States. 
which is perhaps the most numerous of the species in the 
Atlantic States, we have noticed that different localities pro- 
duce very different appearances; even those which inhabit 
different parts of the same river, within a short distance of 
each other, are thus characterized. In the Delaware this 
shell is found sometimes ovate, elongated, cylindrical, ar- 
cuate, and flattened. The species in the Ohio river called 
U. gibbosus, and which is probably the U. fluviatilis, pre- 
sents us also with many varieties of external shape. 
The colour of the nacre of the Fluviatilis is purple, pink, 
or orange, of every shade, and not unfrequently bluish, or 
pearly white. The thickness of these shells differ, some 
specimens are thin and fragile, while others are strong and 
ponderous. In some individuals the periostracha or epi- 
dermis, is marked with broad and narrow dark coloured 
rays, diverging from the beaks,* while in most others it is 
of a plain horn colour, or of a dark brown hue. The beaks 
of the U. fluviatilis have, very frequently, very different 
relative situations — but without entering more at large in- 
to these discrepances, it will be sufficient to remark that it 
would be impracticable to select any specific characters that 
might properly be applied to every individual of this Pro- 
tean race — yet notwithstanding all these anomalies, from 
the vast multitude of specimens which have been obtained^ 
and from the facility with which their habits can be obser- 
ved, little or no difficulty is ever experienced in identifying 
the U. fluviatilis—jQi we may safely say that if but three 
or four specimens of each of the most prominent varieties 
were to be sent to the most eminent conchologist in Europe, 
we should have them returned to us with distinct specific 
* I have noticed the radiating' lines on the U. fluviatilis particularly, 
because the fact has been doubted by some very careful and diligent 
observers. In the nelg-hbourhood of this city these radiated shells are 
very abundant. 
