456 SHIMER. 



It is seen from the above tabulation that it is the brackish water 

 environment which produces the narrow species. This is true, at 

 least, for Buttonwoods and Exeter Street ; we have no data for the 

 Marblehead shell heap. The recent specimens from Marblehead and 

 Dorchester Bay are from normal sea water, as was probably, judging 

 from the associated fauna, also true of the Pleistocene of Sankaty 

 Head, Nantucket. The difference is thus apparently due to environ- 

 ment and is not a permanent change of form due to evolution in time. 



Urosalpinx cinereus. — This species is represented by one individual 

 from Exeter Street, 6 mm. long. The average length of the Muddy 

 River forms is 20 mm., which is likewise the length of those noted from 

 City Point. It was not noted at Berkeley Street. 



Thais lapillus. — The specimens noted from City Point have an 

 average length of 30 mm. The revolving ridges are coarse. 



Tornatina canalicidata. — An average sized specimen from Exeter 

 Street is 2.5 mm. long by 1.5 mm. wide. 



Conclusions. 



The history of Boston from the closing stages of the great continental 

 glaciers covering all of this region to the present day may be summed 

 up in the following five stages: 



1. Deposition in fresh water of mud and sand from the melting 

 glacier; 2. Erosion by streams of some of this material after the 

 disappearance of the glacier; 3. Growth of peat in swampy areas 

 (2 and 3 were probably taking place at the same time as nowhere 

 was peat noted in an erosion channel); 4. Partial submergence of 

 the land beneath the ocean with the accumulation of mud and dead 

 shells upon the peat beds. This record of submergence contains two 

 distinct elements, (a) In the earlier or lower beds the marine shells 

 indicate a warm climate similar to that off the Virginia coast at present, 

 (b) The upper beds, and continuing to the present, where still beneath 

 the sea, contain a marine fauna indicative of a colder climate, that of 

 today. 5. In certain areas, as Back Bay, the raising of the land 

 again from its ocean bed by artificial filling. 



The conclusions bearing upon these five stages are noted below. 



1. The deposition of the blue glacial clay, forming the base of the 

 majority of the sections discussed above, took place probably in a body 

 or bodies, of fresh water, since no remains of animal life are apparent 

 in it. The clay itself, derived from a nearby melting glacier, is the 



