A. C. DiMON 25 



( '(iinpiiriiig in detail tlie liabitfits ul' tlio two spccies, it is sccii that N. obsoleta 

 livc's in lari^c nuniber.s on tiats and .sliorcs Icf't bare ])y the tidi' dui'ing part of thc 

 day, aiul is not f'ound at auy considoiablc depth ; whilo N. trivittata is found 

 in sonio places at a dupth of forty fatlionis, und is not f'onnd abovc low wator line. 

 Verrill ('7.S) statcs that about Vineyaiil Sound jY. ubsoleta ooours in bays and 

 Sounds on sandy and niuddy shorcs and bottonis, and on submorged wood-woik, 

 such as the piles of wharvcs, but that it does not occur on rocky shores and 

 bottoms. N. obsoleta was found in brackish water on sand, nuid, oyster-beds, 

 eel-grass, and subnierged wood-work ; but it was not found in the open ocean. 

 N. trivittata was found by Verrill in all the habitats of N. ohsoleta except the 

 muddy shoi-es of the bays and sounds aud the muddy shores and bottonis, oyster- 

 beds and eel-grass in the brackish water ; it was found in the open ocean on shores 

 and bottoms. With these observations my own expcricnce with the distribution 

 of the two moüusca at Cold Spring Harbor and elscwhere is in complete accord. 



The breeding season of N. ohsoleta is given by Mead ('98) for Woods Hol! as 

 the latter part of April. In July and August when nearly all niy collccting was 

 done there would often be found in pools left abovo low water mark large numbers 

 of very small individuals which were evideutly the brood of that year. The older 

 snails, however, could not be separated into broods of different ages on the basis 

 either of size or the number of whorls, so that eithcr the growth aftcr the first year 

 is extremely slow or eise the snails do not live until the third sumnier. 



(3) Localities froiii which tite sJiells were collected. At Cold Spring Harbor 

 we have, within a small district, several localities in which Nassa may be found. 

 The Harbor is a brauch of Long Island Sound, five miles long by one mile wide. 

 It is divided into an inner and an outer harbor by a sand-spit that extends nearly 

 across it at about half a mile from its head. Tlie inner harbor is fed by a stream 

 which makes its wator decidedly brackish, espocially at the surface. For ncar 

 the niouth of the creek, at the surface, the den.sity may be as low as l'OOG whiie 

 at thc bottom it is I'OIG. Undor other conditions of wind and tide the density 

 will be about I'OIG throughout. The average height of tide is about 7'.5 feet 

 in the inner harbor, and at low tide about half the surface is left bare, exposing 

 flats of black mud on which Ulva grows in abundance. In the outer harbor 

 the density of the water is from I'OIS to 1020. The bottom of the outer barbor 

 consists of mud, oyster-beds or sand, with a good deal of eel-grass ; the shores are 

 Sandy or muddy with a greater slope than those of the inner harbor, so that there 

 are no extended mud-flats exposed at low tide. At the mouth of the harbor, 

 on the east, thc shores are sandy and gravelly and have a considerable slope, and 

 the Situation is far less sheltered than within the harbor. 



The A^assa measured were collected from the three localities marked 1, 2, and 3 

 on the map, Figuve 1. Those from 1 (Laboratory Dock) were gathered from the 

 mud-Üats at low tide ; those from 2 (Laurelton Dock) were taken from the sandy 

 beach at low tide or dredgcd from a few feet of water at the same locality ; 

 and those from 3 (Lloyd Point) were collected from pools left on the sand beach 



Biometrika ii 4 



