BY H. L. KKSTEVEN. 



67 



conch to a calcareous sculptured structure which is, in minia- 

 ture, the same as the adult shell. I regard this last as the 

 Ananeanoconch, and believe that there is no Nepwcoiich 

 present. 



In the case of species in possession of a calcareous shell 

 during the Veliger period, we almost invariably find that 

 there is a varix formed around the peristome of the Jecfo- 

 conch, and the succeeding structure is similar to the adult 

 shell. The assumption of a benthic existence is here, as in 

 the last case, followed by the rapid abortion of the velum, and 

 the varix marks the nepionic pause in growth, is in fact the 

 J\ epioconch. 



In some few cases, however, the prolonged retention of the 

 velum is such -as to allow of the complete development of the 

 mantle-edge and all other organs during the planctonic exis- 

 tence, so that when the benthic existence is entered upon, 

 there is no recognisable pause in growth; the Nepiocoiich, 

 present in these eases, is not recognisable because of its simi- 

 larity to the preceding and succeeding shell-structure. 



In all those forms which pass the whole of the Vehyer 

 stage within the egg-capsule, the whole embryonic history 

 might, in its uneventfulriess, be depicted graphically by an 

 even unbroken curve ; and our problem is to explain why, in 

 view of the unvarying environment, any of the ontogenetic 

 stages should be represented by differentiated shell-structure, 

 rather than to explain why all are not so represented. A 

 quotation from Lankester (35), which I have used before 

 (30), may be aptly used again here: — " ... we may speak 

 of primary, secondary and tertiary shells in molluscs, recog- 

 nising the fact that they may be merely phases fused by con- 

 tinuity of growth so as to form but one shell, or that, in 

 other cases they may be presented as separate individual 

 things, in virtue of the non-development of later phases, or in 

 virtue of sudden changes of the mantle surface causing the 

 shedding or disappearance of one phase of shell-formation 

 before a later one is entered upon." I have elsewhere (29) 



