INSTITUTE OF SCIENCE, PHILADELPHIA. 6 1 



operation resembles a force exerted at the short end of a lever, or between the 

 fulcrum (place of attachment) and the body moved. Any extension of the 

 surface which would give a larger area of attachment or a more efficient at- 

 tachment would in the long run be beneficial to the animal, whose safety de- 

 pends largely upon its being able to withdraw rapidly and completely within 

 its protective shelly covering on the approach of danger. Now, the develop- 

 ment of ridges upon the pillar would give a larger area of attachment to the 

 muscle, would greatly increase the strength of that attachment, and would in- 

 cidentally strengthen the whole shell. Any one who has attempted to remove 

 the soft parts of a Voluia from its shell, will have had sufficient testimony to 

 the added difficulty due to the plications, even in the dead animal, compared 

 with the unplicate Buccinuni or other Rhachiglossate mollusk without plaits. 



In accordance with this hypothesis we find in American rocks the plications 

 first beginning deep in the shell of fusiform gastropods {Piestochil?is, Crypto- 

 rhytis), yet having no plait visible in the aperture, as appears in the later Tire- 

 brispira and Tertiary FasciolaricB . There is some reason for believing with 

 Meek that this succession is also true in Europe and India, but a more thorough 

 knowledge of the fossils is needed before this can be asserted with confidence. 



I should add that the identical principles here invoked will account for the 

 lirae and apertural denticulations of all gastropods, but those about the margin 

 of the aperture are naturally subject to more complex stresses than those deeper 

 within the shell. The inception of deep-seated plaits continuous on the colu- 

 mella, I believe to be due in all cases, as in the present one, to a deeper-seated 

 adductor muscle, operating as above described, and which might be anticipated 

 to occur in widely different groups of spiral gastropods, as, for instance, Can- 

 cellaria among the Toxoglossa, Valuta among the Rhachiglossa, Petaloconclms 

 or Nerinea among the TcEiiioglossa, Actaon in the OpistJiobranchiata, and in the 

 shells of many Pulmonates. The mechanical stresses involved in each case 

 will be the same and followed by similar results, whatever the genetic relations 

 of the particular mollusk. Yet classification hitherto has too frequently been 

 based largely on characters due to temporary stress, while those more deep- 

 seated and obscure, yet fundamental, have been ignored. Of course, characters 

 due to stress have their value, it should not be forgotten ; but the differences 

 of origin of different sorts of characters are yet largely to be worked out and 

 discriminated. 



A point which may be noted in regard to the Volutida: to which my atten- 

 tion was called by Mr. Pilsbry, is that in this group the mantle is greatly ex- 

 tended and there would be more of it to be wrinkled than in such forms as 

 Bnccimim, etc. It may be added that the forms in which we note the beginning 

 of plaits for this family, many of them, such as Liopeplum and Volutoviorplia, 

 had the mantle so extended as to deposit a coat of enamel over the whole 

 shell, as in the modern Cyprcsa, so that here we have an additional reason why 

 plication should be emphasized in this group. 



