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POPULAR SCIENCE REVIEW. 
and the other ( Plectrophrys ) to Pleurophrys. As a negative character, he 
remarks that the Arcdlce, which are so abundant and so easily found in 
fresh water, and the equally abundant and characteristic Diffiugia , are want- 
ing in the salt pond just as in the sea. 
Marine Amoeba. — In the fourth part of the same periodical Dr. Entz 
notes the confirmation of his previous supposition that the Amoeba observed 
in fresh water and in the salt pools, may also occur in the sea. In sea water 
from Cuxhaven he found two Amoeba (A. Umax and radiosa) very abundantly. 
He regards A. marina , Duj. and A. polypodia, Schulze, as probably identical 
with A. radiosa, and possibly also Hackel’s Protamoeba polypodia. 
Young Limuli . — Mr. Alexzander Agassiz writes as follows to u Silliman’s 
Journal”: — “ Mr. C. D. Walcott has called attention to the fact that when 
collecting fossils he finds large numbers of Trilobites on their back.* From 
this he argues that they died in their natural position, and that, when living, 
they probably swam on their backs. He mentions, in support of his view, 
the well-known fact that very young Limudus, and other Crustacea, frequently 
swim in that position. I have for several summers kept young horseshoe 
crabs in my jars, and have noticed that besides thus often swimming on 
their backs, they will remain in a similar position for hours, perfectly quiet, 
on the bottom of the jars where they are kept. When they cast their skin 
it invariably keeps the same attitude on the bottom of the jar. It is not an 
uncommon thing to find on beaches, where Limudus is common, hundreds of 
skins thrown up and left dry by the tide, the greater part of which are 
turned on their backs. An additional point to be brought forward to show 
that theyTrilobites probably passed the greater part of their life on their 
back, and died in that attitude, is that the young Limudus generally feed 
while turned on their backs ; moving at an angle with the bottom, the hind 
extremity raised, they throw out their feet beyond the anterior edge of the 
cenopace, browsing, as it were, upon what they find in their road, and 
washing away what they do not need by means of a powerful current pro- 
duced by the abdominal appendages.” — Amer. Jour . Sci., January 1878. 
* “ Ann. Lyc. Nat. Hist., N. Y.,” Vol. XI., p. 155, 1875. 
eighth Report, N. Y. State Museum,” Dec. 1876. 
u Twenty- 
