128 MOLLUSCA. 



4. Shallow littoral, rocky, protected. East of the un))rotecte(l north- 

 west shore of Stony Island, described under habitat two. and Ijetween 

 that strip and the sandy coast along the north and east sides of the is- 

 land, there is a small stretch of rocky shore protected by a rocky point 

 jutting out to the east of it, and by the shallowness of the water for some 

 distance off shore. Here, the water plants, obtaining a strong foothold, 

 have formed a thick layer of peaty muck over a small area. At present, 

 several species of water plants that flourish here form a small swamp, 

 much like tho.se of the small inland lakes on Sand Point. Scirpus 

 americanus, S. validus. Pontederia cordata and Typha latifolia predomi- 

 nate; Carex filiformis, Sagittaria latifolia and Spartina cynosuroides are 

 also present, but in lesser numbers. 



On driftwood and decaying plant material in this swamp, variations 

 of Lymnaea reflexa were cjuite abundant, together with lesser numl^ers 

 of Planorbis trivolvis, P. exacuous and P. hirsutus. 



5. Shcdloic littoral, sandy, unprotected. This habitat includes all 

 sandy beaches not protected from the direct action of the waves by 

 islands, sandbars, etc. Of course no distinct boundary hne can be 

 drawn between this habitat and the one that includes the protected 

 beaches, as they intergrade around the edges of the barriers. Usually 

 there is no vegetation in the shallow water along the shore in this habi- 

 tat, but in places where the shoals extend some distance out from shore 

 there is a narrow belt of Scirpus americanus along the edge of the water. 

 Being so unprotected, this habitat is one of extremely variable physical 

 conditions; also, the substratum being sand, every storm materially 

 alTects the whole shore-line and the depths of the water thruout the 

 area. This drifting makes a secure footing for molluscs impossible, so 

 that most of the forms in this habitat are quite large, burrowing ones. 

 (Pis. I a, II b.) 



Under this head were included all of the beaches from Ca.seville as 

 far west as Twin Bars (near the end of Sand Point), the shoals northwest 

 of them, and the beaches from about one mile west of Little Oak Point 

 to Hat Point, excepting those on the points themselves, where there are 

 rocky headlands. 



All of the shells of this habitat are bivalves, with the exception of a 

 few which were found on the rocks in artificial breakwaters, etc., and 

 which have been also included among the shells of the rocky littoral 

 habitats, 2, from which they have probably been brought by the waAes 

 and currents. This is probably due to the fact that the univalve mol- 

 luscs are unable to cling to the drifting sand, which would also be liable 

 to bury them on account of their small size. The univalve shells 

 above mentioned were a thin form of Physa ancillaria magnalacustris, 

 which was obtained in considerable numbers from an artificial break- 



