366 W. Upham — Marine Shells and Fragments 



range beyond Cape Cod. In northward range, five extend only 

 to the Gulf of St. Lawrence ; and three of these, namely, 

 UrosaVpinx cinerea, Venus mercenaria, and Ostrea Virginiana y 

 occur only in isolated colonies north of Massachusetts bay. 

 Another, Astarte castanea, has its northern limits on the coast 

 of Nova Scotia and at Sable island ; while the burrowing 

 sponge, Cliona sulphtirea, is not reported beyond Portland. 

 Fourteen are more boreal, of which four continue to Labrador, 

 and ten to the Arctic ocean. 



The great abundance of the round clam, Venus mercenaria, 

 which is now scarce in Massachusetts bay but plentiful south 

 of Cape Cod, indicates that the sea here during part of the 

 epoch just preceding the last glaciation was warmer than at the 

 present time.* Similarly, the colonies of this and associated 

 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, per- 

 mitting these species to advance so far to the north. The in- 

 termingling of characteristic southern and northern forms in 

 this assemblage of fossils 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 boreal and arctic 

 marine fauna. In the Pleistocene beds containing fossil shells 

 on Gardiner's islandf and at Sankoty Head,:}: which are refer- 

 able 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 departure of the last ice-sheet, nor in the 

 scantily fossiliferous continuation 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 



*From the same evidence and the occurrence of other species elsewhere in the 

 Pleistocene deposits of the eastern United States north of their present range, 

 Desor announced in 1847 to the Geological Society of France (Bulletin, vol. v, p. 

 91) and in 1852 in this Journal, (II, vol. xiv, pp. 52, 53) that a warmer 

 climate then prevailed throughout this whole district. 



f Sanderson Smith in Annals of the Lyceum of Natural History of New York, 

 vol. viii, 1867, pp. 14=9-151 ; F J. H. Merrill in Annals of the New York Academy 

 of Sciences, vol. iii, 1886, p. 354, with sections on Plate xxvii. 



\ Desor and Cabot in Quarterly Journal of the Geological Society, London, vol. 

 v, 1849, pp. 340-344, partly quoted by Packard in Memoirs B. S. N. H., vol. i, 

 pp. 252-3 ; Verrill and Scudder in this Journal, III, vol. x, 1875, pp. 364-375. 



