28 



Stiidi/ of xV. ohsoleta and X. triiittata 



The nuinber of whorls was counted directly. Color was ineasiired by the color 

 top in the region E, Figure 2 A ; the results are, however, rather rough. The 

 depth of the suture between the whorls was chosen as an approximate lueaKure of 

 ronghness. It was measured between the last two whorls, near F, Figure 2 a, by 

 laying the shell so that it rested on its aperture and making a camera drawing 

 magnified eight or niue tiuies of the part of the line it w;\s desired to measure. 

 Then the depth of the depression from a tangent drawn to the adjoining elevations 

 was measured on the drawing, and this distauce was dividod by the magnific^ition 

 of the figure. The diniension thus obtained depeuds, however, not solely upon the 

 roughncss of the shell but also upon the convexity of the individual whorls. 

 Figure 3 shows some types of outlines obtained in the nianner described ; 

 h rcpresents au e.xtreme case of a sniooth shell with a deep suture. 



Fig. 3. Camera drawings illustrating tlie depth of suture. S = Shoulder. 



(6) Table üf constants of the curves. In the foliuwing table some of the 

 results of the quantitative detenninations are given. 1, 2, and 3 represent lots 

 of N. obsoleta from the localities referred to ; 3a is the lot ot N. trivittuta from 

 Lloyd Point. 



Discussion of constants. Means. Setting aside for the present N. trivittata, 

 a comparison of the means of JV. obsoleta from the tliree localities brings out 

 certain points. The sheils from 3, the mouth of the harbor, are much longor than 

 the others; those from 1, the inner basin, are the most globose, and have the 

 largest aperture in proportion to thoir size, and the fewest whorls. In dopth 

 of suture the sheils from the mouth of the harbor are e.Ktreme. The probable 

 error of the color measurement is so great that uo stress can be laid on the slight 

 difforences recorded. As to the apical angle, the effect of erosion at the apex 

 upon this reading is, as already stnted, .so considerable that lot 1 (the most eroded) 

 is not strictly companible with the other lots. 



If we coiTelate these dififerences with the environmental differences noted above 

 for the three localities, we find the iudividuals living in less dense water — in the 

 inner harbor — are smaller, more globose, with a larger aperture in proportion 



