ORTMANN: A MONOGRAPH OF THE NAJADES OF PENNSYLVANIA 281 
the water courses are low and clear; but very often there is also a season of low 
water from the end of April to the end of May. In smaller creeks, where the 
water is never very deep, any dry period of about a week’s duration will bring 
about favorable conditions, but in the larger rivers successful collecting is generally 
possible only late in summer and in autumn. The exceptionally dry summers of 
1908, 1910, and partly also of 1909, afforded unusual and unexpected facilities 
for making collections. Collecting in the lakes (Lake Erie for instance) does not 
depend so much on the season, but it depends a good deal on the weather. Quiet 
days, when there are no wind and surf, should be selected. 
Hand-picking is the best way of securing specimens. Devices like the so- 
called ‘‘clam-dredge” of the mussel fishermen of the Mississippi and Ohio are useless 
in our rivers on account of the rough and rocky character of their bottoms. Yet 
in Lake Erie upon sandy and’sandy-muddy bottoms I have used a similar apparatus 
to great advantage. This is a modification of the common “clam dredge,” and 
I am especially indebted to Dr. W. J. Holland for suggestions in constructing 
Fia. 1.. The Holland Clam-dredge. 
this device. It consists (See fig. 1) of a piece of gas-pipe with two wheels at the 
ends, to which four-pronged hooks are attached. The hooks are made of wire, 
with their stems rather long (six to ten inches), and they are tied to the gas pipe 
with very short strings: this prevents the tangling up of the whole mass, which is 
a great nuisance, when, as in the usual clam dredge, short hooks with long strings 
are used. With this apparatus, “the Holland clam-dredge,”’ I obtained shells in 
Presque Isle Bay, Lake Erie, at depths of from six to fifteen feet. 
In 1910 I had special advantages for collecting Unionide in Lake Hrie, by 
being offered the opportunity to go out with the ‘‘sand-sucker” of the Erie Sand 
and Gravel Co. I thus obtained specimens at depths of from ten to fifteen feet. 
