412 PEOCEEDINGS OF THE ACADEMY OF [1896. 



soft parts, so that if the suggested growth be sudden, as if forming 

 a climax during which maturity is rapidly assumed (a state of things 

 readily induced by changes in the reproductive organs and the ripen- 

 ing of their contents), a sudden change in the direction and form 

 of the whorl may be induced dynamically. This is what I believe 

 takes place in such forms as Holospira, Cylindrella and various 

 Cydostomatldce. If we picture the animal on a twig, holding on by 

 the foot and partially retracted, the spire heavy with contained ova 

 and the animal at rest, pending secretion of shell matter, it is easy 

 to imagine the manner in which the mature aperture may be built 

 up on the margin of a perpendicularly pendant immature shell, 

 without following the cycloidal curve of the earlier whorls. 



The influence of a very dry warm atmosphere on the expanded 

 mantle will be analogous to that of alkaline matter, but likely to 

 act with less irregularity. A particle of alkaline dust might affect 

 a small part of the margin of the mantle and not the rest, while the 

 air might be expected to act on the whole expanded margin. It is 

 probable even then, however, that some portions of the edge might 

 dry quicker than others and more or less irregularity would almost 

 certainly result. Of course, if the margin of the mantle were to be- 

 come actually desiccated, secretion would cease and could not go on 

 again until the dry tissue had been cast off and replaced. But it is 

 probable that the tissue is too sensitive for such an event to occur 

 under ordinary conditions. It would probably ojierate so that when 

 the animal felt the mantle becoming uncomfortably dry, it would 

 simply retract, and temporarily cease secretion as in the presence of 

 alkali. But enough has been said to indicate the mode by which 

 drought and alkaline matter may act upon the growing mollusk and 

 directly modify its secretions, and, by consequence, its hard parts. 

 That this action takes place substantially as suggested I have little 

 doubt, and that its results may be differentiated from those of nor- 

 mal growth in continuously favorable conditions, I think will be 

 shown to be probable. 



Let us consider the features presented by Bulhnulus Simrothi and 

 see how far they exemplify the processes above described. The de- 

 position of ova may take place with the opening of the wet season. 

 No data are available, but none of the specimens collected in the 

 hibernating state by Dr. Baur, and of which the soft parts were ex- 

 amined, contained any developed ova. It would be in accordance 

 with what we know of species in other regions if the ova were rapidly 



