452 SHIMER. 



Many plantings of the oyster spat in its old home in the Charles 

 River during recent years have resulted merely in the death of the 

 spat. 



Pecten gihhus borealis. — (This is our common scallop, the Pecten 

 irradians of authors.) The single specimen seen from Muddy River 

 is 26 mm. long by 25 mm. high. It is one of the common forms at the 

 Charles River locality. 



Mytilus edulis. — The edible mussel is present though apparently 

 rare at Berkeley Street and City Point. 



This species, occurring from about half tide down into comparatively 

 deep water, was very abundant during early colonial times and was 

 largely eaten by the colonists. Since it is a rather open coast form 

 its rarity at the Back Bay localities is not surprising. 



Modiolus modiolus. — This deep water inhabitant is represented in 

 our Exeter Street collections by but one valve, 3 mm. long. 



Modiolus demissus, var. plicatulus. — This is the coarse horse-mussel. 

 It is very abundant in the superficial sediment at Muddy River. 

 This mud was formed after the typical marine shells had been deposited 

 and is hence comparatively recent. This species of Modiolus continued 

 to thrive here until the completion of the dam across the tidal portion 

 of the Charles River in 1911. Only a few, but characteristic, pieces 

 of this shell were noted at Exeter Street. 



Venus mercenaria. — The Muddy River forms are normal in size and 

 weight; the concentric growth lines are quite strong. The Charles 

 River and Berkeley Street specimens are similar to these and are 

 very abundant. 



The quahog, little-neck, round or hard clam, is now rare north of 

 Cape Cod, as it apparently was during the early colonial days. 



Gemma gemma. — This is the most abundant form found at Exeter 

 Street; a specimen of average size measures 3+ mm. in length by 

 3+ mm. in height. It is likewise very abundant and of a similar 

 size at Clarendon and Berkeley Streets. 



The average of 10 specimens from Exeter Street gives a proportion of 

 height to length of 1 to 1.06. The average of a similar number from 

 Provincetown, Massachusetts is 1 to 1.15. 



This greater height of the subway forms may be interpreted either 

 as an evolution toward greater length since that time or as evidence 

 of a slightly more unfavorable environment in the Back Bay area. 

 This latter hypothesis is partly corroborated by the specimens of the 

 same species from Buttonwoods, Rhode Island. Here, far up the 

 Narragansett Bay, the average of height to length is 1 to 1. 



