2 SMITHSONIAN MISCELLANEOUS COLLECTIONS [VOL. 50 



Family VOLUTIM! 



The inception of the Volutidae, Fasciolariidae, and Turbinellidse 

 appears to have begun in the Cretaceous from a stock of Proso- 

 branchiate Gastropods apparently also the progenitors of another 

 series in which plaits were not developed on the pillar. I have else- 

 where described the dynamic principles concerned in the develop- 

 ment of plaits in spiral shells of any genus, 1 and it is only necessary 

 to recall the fact that the horizontally or obliquity of the plaits is a 

 function of the plane of enrollment of the whorls, more or less modi- 

 fied by the shape of the aperture and canal. Other things being 

 equal, the shell whose whorls are coiled most nearly in the same 

 plane will have the most nearly horizontal plaits. 



To small forms which illustrate the inception of plaits upon the 

 pillar, as would synthetic types of the family groups above referred 

 to, Meek gave the name of Piestochilus. 2 Another form, called by 

 him Mcsorhytis, and still persisting in the deep-sea fauna, is referred 

 by many paleontologists to the genus Mitra, and may be more closely 

 related to the Mitridse than to the group we are discussing. 



The forms which appear early, and in which the generic 

 type seems hardly settled into equilibrium, are usually lumped by 

 authors under the inappropriate name of Volutilithes; the true Volu- 

 tilithcs having a different development, a membranous instead of a 

 shelly protoconch, and first appearing in the Eocene. The antithetic 

 genus, Plejona (Bolten) Dall, is more closely related to these Cre- 

 taceous types from which it is no doubt descended. The species 

 which may be properly associated with Plejona among these early 

 mutable types are those which have an excavated columella with an 

 anterior heavy plait, behind which may be several smaller and less 

 distinct plications. 



The forms which are developing in the direction of the Volutidse 

 of the future and which first show the Volutoid characteristics 

 appear in the middle Cretaceous, and it is this line of evolution 

 which this discussion is intended to follow. 



1 Am. Naturalist, xxvin, Nov., 1894, pp. 909-914, figs. 1-3 ; see also Trans. 

 Wagner Inst., ni., p. 58, 1890, et seq. 



- Smithsonian Check list N. Am. Cret. Foss.. p. 22, 1864. Since the species of 

 Piestocheilus named by Meek come from high up in the Cretaceous, while the 

 most nearly related American Volutes come from the Pugnellus sandstone 

 (Turonian?), it is not intended to regard the former otherwise than as later 

 representatives of Mid-Cretaceous forms which, through the imperfection of 

 the geological record, are yet unknown to us, but presumably resembled 

 Piestocheilus. 



