BY n. L. KESTKVEN. 67 



will have a larger diameter than the succeeding whorls. If, 

 on the other hand, the plan of coiling of the shell be a close 

 spiral without an umbilicus, then if the conditions found in 

 Spirilla are present, the caliculus must be placed more or less 

 eccentrically. 



Whence it may be stated that a large caliculus placed 

 eccentrically, owes its origin and eccentricity to (1) the size 

 of the egg and larva during its formation, and (2) the rate 

 of growth of the larva subsequent to its formation. 



Isolated examples of the condition in question could pro- 

 bably be found in almost every genus among the Gastropoda ; 

 to attempt to make a complete list would serve no good 

 purpose. I give but a few typical, instances: — Scaphella 

 (Voliita) mamilla Gray, Columhelhi australis Gaskoin (26), 

 Cymatium {Triton) woodsi Tate and C. radiale Tate (28), 

 Megalatractus art/anus Linn. (31), Triphora lahiata A. 

 Adams (13), Mangclin lutrar'in Hedley (14), Gerithiopsis hal- 

 hgani Hedley (15), Thetldos morsura Hedley (16), Turricula 

 pilshryi Hedley (Joe. cit.), Syrnola macroc-ephala Hedley (17^, 

 Mathilda decorata Hedley (loc. cit.), Cerithiopsis cacumina- 

 tus Hedley and Petterd (18), Pleurotoma casearia Hedley 

 and Petterd (loc. cit.), Terchra lauretantt Tenison- Woods 

 (loc, cit.). This list might be further added to without going 

 outside the limits of my own library, which is quite small. 

 Two facts stand out from it — (1) that the large apex occurs 

 in genera in which, as a rule, the apex is small and quite 

 symmetrical ; (2) that it occurs more frequently in deep water 

 than in littoral forms. 



The above are instances of a particular kind of apex arising 

 out of particular and recognisable morphological features 

 of ovum and larva, but, as yet, no explanation has been 

 offered for those morphological features ; that explanation 

 may perhaps be extracted from the following observation by 

 Sturany. In Fusiis hifrons and its variety paucicostata two 

 types of protoconch occur ; in the type, which inhabits shallow 

 coastal waters, the protoconch is small ; in the variety, which 

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