72 THE CONSTITUTION OF THE GASTROPOD PROTOCONCH, 



related to the outer aspect of the last whorl, and the muscle 

 attached to the colunaella (28). 



Suppose now, in the latter form, the mollusc, already in 

 possession of all its adult characters, becomes rotated within 

 the shell through an angle of 90°, so that the dorsum be 

 related to the lower wall of the ultimate whorl, as is the 

 case in Eisella, the shell subsequently formed will have its 

 axis at right angles to that of the portion formed before the 

 rotation, and there results a dextral heterostrophic apex. In 

 the true Heterostrophe apex, the process is further compli- 

 cated by the sinistral coiling of the shell previous to the 

 rotation. 



In the Anastrophic apex (Solarium), there is a simple 

 change from sinistral to dextral coiling without any rotation 

 within the shell ; had such occurred in the elongated, closely- 

 coiled Eulimoid and FyramideUoid species, then in these 

 also the axis of the apex would have been that of the adult 

 shell, and the latter would be wound round the sinistral apex. 

 The heterostrophic apex then may be said to have resulted 

 from two factors, a sinistral apex, and growth of the dorsal 

 portion of the body so much in excess of the ventral as to pre- 

 clude an open coiling of the shell, such as would permit the 

 inclosure of the sinistral portion on the assumption of the 

 dextral coiling, and this latter evoked a rotation within the 

 shell. It is interesting to note that, even in those heteros- 

 trophic apices of one whorl, which would not have interfered 

 with a dextral coiling without rotation, the rotation has 

 nevertheless occurred. 



The Ananeanoconch. 



In view of the demonstrated fact that some protoconchs 

 are formed in part subsequent to the nepionic stage, such 

 protoconchs are of varying significance. Two more or less 

 sharply differentiated types of Ananeanoconch are recognis- 

 able ; those which form the initial portion of the true conch, 

 and present more or fewer of the adult characters ; and those 



