1888.] 
135 
[Upham. 
dated southern species, scattered here and there northward to the 
Bay of Chaleurs, are evidence that since this last glacial epoch the 
sea has been again warmer than now along this coast, permitting 
these species to advance so far to the north. The intermingling of 
characteristic southern and northern forms in this assemblage of fos- 
sils from the till seems to be readily accounted for by the gradual 
refrigeration of climate which culminated in the formation of the ice- 
sheet. Before that time the round clam or quohog and other shells 
of chiefly southern range were doubtless succeeded by a wholly bo- 
real and arctic marine fauna. In the Pleistocene beds containing 
fossil shells on Gardiner’s island 1 and at Sankoty Head , 2 which 
are referable to the same epoch with these near Boston, namely, 
the interglacial epoch preceding the latest glaciation, the round clam 
occurs in abundance ; but it has not been discovered fossil north 
of the sections here described, which indeed are the most northern 
yet found in northeastern America holding fossils of interglacial 
age. It has not been found in the plentiful fauna of the marine 
beds of modified drift deposited in southern Maine during the de- 
parture of the last ice-sheet, nor in the scantily fossiliferous contin- 
uation of these beds southward to Portsmouth, Gloucester, and 
Cambridge. 
Nearly all the species of our list inhabit the shore or shallow 
water, from low tide to the depth of a few fathoms, though some of 
these also range downward to considerable depths. Three, of which 
two are doubtfully determined, are probably restricted to compara- 
tively deep water ; but even these are often cast ashore in severe 
storms. Considering the outlines of our eastern coast and the di- 
rection of the motion of the ice-sheet, it seems probable that these 
fossils were living along the shore and in the shallow edge of the 
sea on the area between the mouths of the Charles and Saugus 
rivers. In that interglacial epoch the drumlins of this district had 
not been accumulated, and the greater part of Chelsea, Revere, 
and Winthrop, formed of these and other deposits of glacial and 
modified drift, may then have been sea of similar depth with the 
present harbor of Boston or the part of Massachusetts bay between 
1 Sanderson Smith in Annals of the Lyceum of Natural History of New York, vol. 
viii, 1867, pp. 149-151 ; F. J. H. Merrill in Annals of the New York Academy of Sciences, 
vol. iii, 1886, p. 354, with sections on Plate xxvii. 
2 Desor and Cabot in Quarterly Journal of the Geological Society, London, vol. v, 
1849, pp. 340-344, partly quoted by Packard in Memoirs, Boston Soc. Nat. Hist., vol. i, 
pp. 252-3; Verrill and Scudder in Am. Journ. Sci., ill, vol. X, 1875, pp. 364-375. 
