Alona falcata—Alonella rostrata. 
305 
Alona falcata, Sars. 
Plate XIII, Figs. 9, 10. 
, This species was found not , very rare ifi dredgings from shal¬ 
low water in Washington Harbor, Isle Royale and in lake 
Oogebic, Michigan. There are some minute differences between 
my specimens and some of the European descriptions. Sars’ 
account (’62, p. 162,) is more like our form than are those of 
P. E. Mueller (’68, p. 183,) or Hellich (’77, p. 95). Both of 
these authors are less accurate in their account of the posterior 
margin of the valves than is the original description of Sars. 
There is a distinct dorso-posteal angle, and the margin is 
sinuate. The infero-posteal angle is rounded and the teeth, 
which number 1-3, are set, not on the margin but a little 
within it. The cilia of the ventral margin are long, especially 
at the anterior and posterior ends. 
The post-abdomen is well described by Sars as “crassum fere 
teres. ” The dorsal lower margin is carried almost parallel to 
the ventral margin of the valves and the organ is used in loco¬ 
motion somewhat as it is employed by Dunhevedia. 
Alonella rostrata, Koch. 
1878, Birge, Pleuroxus acutirostris ; p. 23, pi. II, f. 15. 
This species was found at Minocqua, and more abundantly 
from Isle Royale and also in collections from Easthampton, 
Mass. It has not been recognized before in this county al¬ 
though I earlier described it as a new speciec of Pleuroxus • Its 
affinities are more nearly wijh Pleuroxus than with Alona. The 
single spine of the postabdomen is the chief character in com¬ 
mon with Alona , while shell markings and beak are Pleuroxine. 
I see no ground for including the forms with short beaks —- 
P. excisus and exiguus — in the same genus with this distinctly 
long-beaked form. 
