MOLLUSC A. 143 



moist places (many of which are flooded in the spring) the land shells, 

 especially the smaller ones, are quite numerous, surpassing in numl^ers 

 of species, altho not in indi\'iduals, the sedge-heaps along the beach. 

 These localities vary considerably in the numl^ers and habits of the 

 shells present, as the concUtions vary from those in small hollows in the 

 sand dunes, which are flooded in the spring and dessicated in the sum- 

 mer, to the large, moist alluvial flats and cedar thickets along the lakes ^ 

 and marshes. (PI. XII b.) 



In this habitat, Zonitoides arborea, Z. nitida. Vertigo ovata, Agrioli- 

 viax campestris, Polygyra alholabris, P. monodon, Succinea retusa, S. 

 avara, Carychium exile and C. exiguum were collected in considerable 

 numbers. Bifidaria tappaniajia, B. contrada, B. pentodon. Punctum 

 pygmaeum, Helicodiscus parallelus, Vitrea rJwadsi, V. indentata, V. ham- 

 monis, Euconulus jidvus, Pyramidula alternatu, Polygyra thyroides, P. 

 multilineata, P. alholabris dentata, Vertigo gouldii, V. ventricosa elatior, 

 Succinea oralis, and Philomycus caroUnensis were also obtained, but in 

 lesser numbers, the last nine being represented by only a few specimens. 



Along the borders of bodies of water, these habitats are constantly 

 l^ecoming larger, owing to the fiUing-in of the swamp or lake which they 

 sm-round, while the xerophytic conditions are, in their turn, encroach- 

 ing on these more humid ones: thus the latter displace the aquatic 

 habitats, and are themselves finally, but much more slowly, destroyed 

 by the advance of the sand dune conditions. 



27. Under rock ledge. Hat Point. Hat Point, as mentioned under 

 preceding habitats, is formed by an outcrop of Marshall Sandstone 

 which juts out over the water, forming a small cliff some fifteen or 

 twenty feet high. The waves have worn this away at the base so that 

 it hangs over the water and small caves are formed. Along both sides 

 of this Point, the sand has iDeen washed and blown up around it; on the 

 west side it is flush with and covers the rock, but on the east side the 

 small overhanging cliff is still exposed for about one hunched and fifty 

 yards back from the Point. Against the base of this, the fallen leaves 

 of the nearby birches (Betida papyrifera). the red oaks {Quercus rubra), 

 and the dogwoods {Cornvs stolonifera and C. cireinata) have gathered, 

 and the ground is kept moist and well shaded by the trees themselves 

 and the overhanging rock, thus forming an excellent habitat for the 

 small shells. 



In these places, a few specimens of Vitrea binneyana, Helicodiscus 

 paraUelus, Zonitoides minuscula, Z. arborea, Pyramidula cronkhitei 

 antJwnyi, P. c. catskillensis, Sphyradium edentulum, Euconulus fulvus, 

 Strohilops virgo. Agriolimax campestris, Pallifera dorsalis, and Polygyra 

 thyroides were collected. 



28. Outer sand dunes. The outer sand dunes form the most un- 



