58 TRANSACTIONS OF THE WAGNER FREE 



the Mesozoic Fiisidcs are descended, on the one hand, Tiidicla, Fulgur, Fascio- 

 laria and Latinis ; and, on the other, Voliitomorpha, Vohitoderma, Vobitilitlics, 

 Lyria, Valuta, Caricella, Scaphella, Aiirinia, TurbincUa proper and Vasiun. ■ I 

 have not investigated the relations of Marginella sufficiently to venture on an 

 opinion as to its descent. 



Origin of tlie Plaits. — The question which first arises is as to the origin of 

 the columellar plications and their function. In considering the dynamic re- 

 lations of the animal to its shell we may obtain satisfaction on this point. In 

 the fusiform rhachiglossa an anatomical difference exists to which I believe at- 

 tention has not hitherto been called. Indeed, unless the principles of dynamic 

 evolution are granted, it is a difference which would appear to have little or no 

 significance. These principles, however, afford a key which seems to unlock 

 this and many other mysteries. In the recent forms of this sort the adductor 

 muscle, which in all gastropods is attached to the columella at a certain dis- 

 tance within the aperture, is attached deeper ivitJiin the shell 'Ca'a.n in non-plicate 

 forms. The point of attachment may be an entire turn or even more behind 

 the aperture, while in short globose few-whorled shells and in the non-plicate 

 forms it is, as a general rule, little more than half a turn behind the aperture. 



Now let us consider the dynamics of the case. We have, reduced to its 

 ultimate terms, a twisted shelly, hollow cone, subangulate or even channelled 

 at two extremes corresponding to the canal and the posterior commissure of 

 the body and outer lip. Inside of this we have a thin, loose epithelial cone, 

 the mantle, of which the external surface, especially toward the margin, is 

 sheli-secreting ; lastly, inside of the mantle-cone we have a more or less solid 

 third cone, consisting of the foot and other external parts of the body of the 

 animal, which can be extended beyond the mantle-cone outwardly, as the man- 

 tle-cone can be beyond the shell-cone. The body-cone and the mantle-cone 

 are attached at one of the angles of the shell-cone some distance within the 

 opening of the spiral of the latter. The two outer cones constitute a loose, 

 flexible funnel within a rigid, inflexible funnel, while the body-cone forms a 

 solid, elastic stopper inside of all. 



What will happen according to mechanical principles (which can be tested 

 by anybody with the simplest apparatus) when the mantle-cone is withdrawn 

 into a part of the shell-cone too small for the natural diameter of the contracted 

 mantle-cone ? It must wrinkle longitudinally. Where will the wrinkles come ? 

 They will come at the angles of the shell-cone first ; they will be most numer- 

 ous toward the aperture, since toward the aperture the mantle-cone enlarges 

 disproportionately to the caliber of the shell, owing to its processes, the natural 

 fold of the canal, etc., etc. ; the deepest and strongest wrinkles will be concen- 

 trated on the pillar, owing to the fact that the attachment of the adductor pre- 

 vents perfect freedom in wrinkling and the groove of the canal will mechanic- 

 ally induce the first fold in that vicinity. The most numerous small wrinkles 

 will be near the aperture opposite the pillar, because of the mantle-edge this is 



