A. C. DiMON 



;{i 



average betwecn thc dimonsions giveii l)y Apgar aiid Guuld as rcpresentiiig Ihc 

 normal type of iVas.v«, loiiiid in innre cxiinsed situations than Cold Spring Harbor. 

 The second and fourth columns of Table II. give this average for N. obsoleta aiid 

 N. trivittata respectively, while the first colunin gives an average of tho nioaiis 

 of N. obsoleta from localities 1, 2, and 3, and Lhe third column gives the nieans 

 of N. trivittata from 3. Table II. shows that the Cold Spring Harbor slielLs of 

 N. obsoleta are plainhj clepaiiperate, — smaller, more globose, and with a larger 

 aperture and fewer w/ioiis than thc nornud. All these signs oj depauperizatmi mai/ 

 be associated with the lack of density of the water, and the sheltered Situation as 

 coinpared tuith the open ocean. 



TABLE II. 



Comparison of Cold Spring Harbor type of snails with the normal type. 



This depauperization is evident in the case of N. trivittata as well as 

 K obsoleta, and may be explained in the sanie way. Long Island Sound, as 

 a whole, presents conditions different from those found in the open ocean, its 

 water being less dense on account of the rivers that enipty into it,and its Situation 

 more sheltered. N. trivittata, in foct, is found much niore abundantly in the open 

 ocean, and the locality from which thc lot measured was collected represents the 

 furthest limit of its intrusion into sheltered, relatively fresh water, in which rcspect 

 it may be compared with locality 1 as a habitat for iV. obsoleta. It is what we 

 should expect, therefore, that the N. trivittata from Cold Spring Harbor shoiv the 

 sanie relation to normal iV. trivittata as the N. obsoleta from locality 1 shoiu to 

 normal N. obsoleta. 



(8) Coeßcients of variability. In considering the coefficients of variability 

 it must be kept in mind that comparisons must be niade with caution. Thus, 

 for e.xample, the variability of red is given in Table I. as from 17 °/^ to 25 7,^, and 

 the variability of black as from 2 7„ to 3 7„. It is obviously incorrect to say that 

 red is from eight to twelve times as variable as black. so that here the coefficient 

 of variability has evidently a different significance in the case of the two colors. 



