A. C. DiMON 



27 



the determinatioiis of the nuiiibor of whorls, oolor, and roughness wcrc made oiily 

 once, so thcse dutermiiiations ai-c less accurate. To measuro length a niicrometor 

 gauge reading to hundredths was used. In Figiire 2 a, the line AD represents the 

 diroction in which length was measured, B the direction for diameter, and C for 

 greatest length of aperture. The sliape of the shell was given by the angle 

 at the apex and the ratios of the diameter to the length and of the aperture 

 to the length. The ratios were calculated fnmi the nieasurements ; the angle 

 was measured directly by nieans of a bevel protractor. The erosion of the apex 



Fio. 2 a. 



Fia. 2 B. 



disturbed the measurement of the angle in the case of N. obsoleta, and it was 

 finally decided to takc an angle the direction of each side of which would be a 

 compromise between the directions of lines drawn between the center points of 

 successive whorls. D, Figure 2 a, shows such a compromise between lines 1, 2, 3, 

 on one side, aud 4, 5, on the other, and represents what I have called the apical 

 angle. Even after the niore eroded shells had been discarded, the bluntness 

 of the apex often affected the angle, the geueral tendency being to read it too 

 large, as showu in Figure 2 B. The niean angles of shells fi-oin the different 

 localities are therefore to be compared only with caution. 



4—2 



