26 



Study of N. ohsoleta and N. trtvittata 



at low tidc. It was not possible to find N. trivittata in abundancc in all thrce 

 localities, so this species was taken only fi"om 3, where the shells were washed up 

 on the beach in considerable nunibers and easily collcctcd. Only one or two 

 specimcns were di'edged l'roni locality 2, and no specimen has been found in the 

 inner harbor. 



Fig. 1. Sketch Map of Cold Spring Harbor. 



(4) Qualitative comparison of the shells Jrom the different localities. If 

 hauflfuls of N. ohsoleta from each of the thrce localities niarkcd on the map 

 be compared a decided diffcrencc in sizc will bc at once noticcd. The shells 

 from 3, the most exposed locality, are much the largcst. On the other band 

 they are niiich niore nunierous in the inner harbor. In comparing N. trivittata 

 witli N. ohsoleta it is seen that the shells of N. trivittata are not covercd with 

 algae and are not erodcd, while the shells of N. ohsoleta are covered by algae and 

 much eroded at tip, probably in consequence. Also lY. trivittata is almost white, 

 whereas N. ohsoleta is blackish jnirplc, its apical angle is morc acute, and its shcll 

 is smaller. 



(5) Method of measuHng. The characteristics of the shells of which it was 

 sought to get a quantitative expression were size, shape, number of whorls, color, 

 and roughness of surface. To ensure accuracy the dinieiisions of the shell and 

 the angle at its apex were all measured two or three tinies. On accoiint of the 

 length of the process or the great effect of a previous reading on the judgment 



