Manchester Mejnoirs, Vol. Ivii. (191 3), No. 10. ii 



tions occur in shells which are almost discoidal, and in 

 those with high spires. Not only does variation occur 

 among different individuals, but considerable changes 

 may take place during the growth of a single individual, 

 as in the example shown in PL /., Fig. 3 E, where the 

 apical part of the shell is almost normally involute, while 

 the lower portion is much less involute. Similarly, the 

 apex may be sub-normally involute, while the later parts 

 are normal, and so on. Marked variations of the type 

 just described within the individual are relatively in- 

 frequent. Another individual variation in involution is 

 almost constant, namely, the terminal portion of the last 

 whorl of the mature shell is nearly always somewhat 

 " turned down," so as to be less involute than the pre- 

 ceding portions. 



(3) SJioulders and Chatinelled .Sutures. These features 

 vary generally in relation with the degree of involution. 

 When the involution is normal, the upper flank of each 

 whorl is nearly flat, with its edge only very slightly in- 

 curved where it joins the whorl above (see, eg., PI. /., 

 Fig. 3 C). In those cases in which the overlap of the 

 whorls exceeds the normal, the sutural margin of the 

 upper flank of each whorl is strongly incurved, producing 

 a "channelled suture," while the adjacent portion of the 

 flank forms a slrongly arched, prominent "shoulder" (see, 

 e.g., PL /., Fig. 3 A and B). The upper flank of the whorl, 

 between the shoulder and the dorsal carina, is in these 

 cases slightly concave in form. On the contrary, when 

 the overlap is less than normal, there is generally no 

 shoulder or channelling, and the upper flank tends to be 

 convex (see PL /., Fig. 3 d). The relation of these 

 variations to the involution, though general, is by no 

 means constant. It should be noticed that the form of 

 the upper flank of each whorl is modified not only by the 



