510 PROCEEDINGS OF SCIENTIFIC SOCIETIES. 
The ice thawing, the detrital matter which lay beneath it, and is now 
known as typical drift, would be laid bare and left substantialy as we 
find it. In this view a resort to a depression of five thousand or six 
n 
? 
icebergs could not furnish the material of New England typical drift, 
since it is for the most part of local origin; while bergs of ice from the 
White Mountains could not have supplied it, for it is a continuous sheet, 
far to the.north of these mountains. So iceberg 
posited it, because, as they slowly wasted, the particles of matter must 
have been scattered by the flux and reflux of the tides, and thus to a large 
' extent stratified. Again, from the southern border of the wasting ice- 
laying down those deposits known as modified drift. These constitute in 
part the terrace formations, which usually slope with the rivers along 
which they occur. In some instances there were barriers obstructing the 
waters; thus were formed ponds and lakes, in which deposition took 
place in more nearly horizontal layers. Finally from the wastin 
ice-sheet the surface of the ocean must be elevated, its waters spread 
in which we'now find them. 
In conclusion it may be asked whether the explanation suggested be 
not in consistency with the facts, and thus whether we ought not to ac- 
r. described three new generic forms of Brachiopoda, princi- 
pally ffom the collections of the United States Exploring Expe ition. 
Two of these o the group of articulated Brachiopods, while 
the third was that animal, which, under the name of Lin 
been described by Mr. Morse. Mr. Dall then spoke of several special 
points of structure, especially the peduncle of Lingula, demonstrating its 
construction to be analogous to that of the siphons of bivalve mollusks, 
as the common clam, Mya arenaria. He then described the bristles 
io 
Mr. Dall took the opposite view, and, while admitting all the facts 
