POST-GLACIAL HISTORY OF BOSTON. 455 



Paludesirina minuta. — The Exeter Street specimens are small; 

 a common form has a length of about 2.5 mm. and a width of 1.5 mm. 

 The average of five specimens from here gives a proportion of greatest 

 breadth to -length of 1 to 1.78; of five from Dan vers, Massachusetts, 

 1 to 1.68, an increase in breadth of the living individuals. 



Bittium plteniatum. — -One individual was noted at Exeter Street; 

 this is 6 mm. long and retains a few patches of its original slate color. 



Triforis nigrocinctus. — Only one example, 3 mm. long, of this 

 sinistral, granulated shell was noted at Exeter Street; it has become 

 an ashy gray except where between the ridges it still retains some of 

 the original dark red color. 



Nassa trivittata. — ^The specimens from Exeter Street average 

 8.3 mm. in length by 4.8 mm. in width. Recent specimens from 

 Ipswich Beach, Massachusetts, average 25 mm. by 9 mm. Living 

 forms have thus attained a larger size and a greater proportionate 

 width, averaging 1 to 2.8 as against 1 to 1.73 in the Exeter Street 

 forms. 



Ilyanassa obsolcta. — The forms from Exeter Street are about 

 half the size of the normal species of this coast, averaging 12 mm. in 

 length. The costae are likewise stronger than on the normal shell, 

 approaching Urosalpinx cinereus in this respect; there are 10 to 17 

 costae present. They are most similar in every respect to the ones 

 living at Buttonwoods, west of Warwick Lighthouse, far up the 

 western side of Narragansett Bay, both probably owing their small 

 size to the freshened condition of the water. The Muddy River 

 forms are much larger, averaging in length 22 mm. 



The following list of average measurements compares in size and 

 proportion specimens of this species from several fossil and recent 

 localities : — 



Length Width Proportion of 



Collected by Professor E. S. Morse. 



