1886.) Js Littorina litorea Introduced or {Indigenous ? 935 
the coast of Maine about 1868; Dr. Dawson, however, states that 
he collected it on the shores of Nova Scotia at a much earlier 
date. I wish at present merely to put on record some additional 
data as to its recent progress along the coast. In 1873 it was 
mission, and was found sparingly at Peake’s island, Casco bay. 
n 1872 it was very rare at Provincetown, Mass., but in 1875 it 
Was common there. In 1875 it was collected by the writer at 
Barnstable, Mass., on the shores of Cape Cod bay, in large quan- 
tities. In 1879 it had become exceedingly abundant at Province- 
town. In 1875 our parties found two specimens only on the 
Southern shores of Cape Cod, at Wood’s Holl, but in 1876 it was 
found to be common there, and is now very abundant. The first 
Specimen found so far westward as New Haven was obtained by 
‘Professor S, I. Smith during the past winter [’79-80]. Other sol- 
itary specimens have since been obtained here by E. A. Andrews 
| and by J. H. Emerton. It is at present exceedingly abundant at 
Newport, R E” 
It is spreading into the Gulf of St. Lawrence, too, finding prob- 
ably a congenial habitat in the warmer water of Northumberland 
straits, which contain so many southern forms. J. F. Whiteaves 
found it at Souris and Charlottetown, P. E. I., in 18732 
Do not these facts afford an exceedingly strong argument that 
the shell has been introduced? Its rapid increase southward 
Shows that a favorable habitat was there waiting for it—a much 
more favorable one than the Nova Scotia coast. The conditions 
Which determine its spread were here af work a century ago, but 
it 
Was not found anywhere in New England. 
As has already been pointed out, no species of animal or plant 
‘an be truly indigenous to the two continents. Tt must either 
ve originated in one and spread to the other, or it must have 
originated at some other point and spread to both. A shell such 
a we are considering, which is at present common to both con- 
tinents must either have been introduced from one to the other 
-Y man’s agency, or by purely natural means. If it can be shown 
lat the natural means did not operate in this case, it would prove 
at Man must have introduced it; and the stronger the proba- 
> ity of the former, the stronger will be that of the latter. 
— Winds or the agency of birds, so active in the distribution of 
Plants, could hardly operate upon a shell or its young. Ocean 
A Seem to be the only method of conveyance. But by no 
means could either L. Ætorea or L. palliata directly cross the At- 
*Report on deep-sea dredging operations in the Gulf of St. Lawrence. 
3 62 
VOL. XX.—xN0. x1. 
