64 TRANSACTIONS OF THE WAGNER FREE 



line of Scapliella proper as typified by S.junonia. This group in the Eocene 

 comprises short, bulbous forms like C. prisca or C. doliata, almost devoid of 

 spire; and long, slender, mitriform species like C. demissa and C. Leana. The 

 short forms have nearly horizontally coiled high plaits like those of Tiirbi- 

 nella, and were therefore referred to that group as a subgenus, by Conrad. 

 The longer forms have more and more oblique plaits. The Turbinellas, an 

 offshoot from the early Voltitidce {Latinis and its allies excluded), have a very 

 large, stout column, and therefore nearly horizontal plaits, which are character- 

 istic of the genus. Aurinia ditbia, which has an extremely slender axis, has the 

 plaits correspondingly oblique. The Mitras, which have many turns in a 

 given length, are notable for their horizontal plaits. The few species which 

 have few whorls have more oblique plaits, and one of the finest examples of 

 this sort in the genus Mitra was referred to the Vobitidcz by Conrad on that 

 ground, under the name of PUioptygma. 



In species whose body when retracted solidly fills the whorls, the coil of 

 the plaits must, as it always does, conform to the general spiral of the whorls. 

 In species, of which there are a few, in which the body is peculiarly slender, 

 though an expansive mantle forms a capacious whorl, the plaits resulting may 

 be more oblique on the pillar than is the general coil of the spire, and thus a 

 few apparent exceptions are accounted for. This is a matter of observation 

 as well as theory. 



Number and Form of the Plaits. — In the first initiation of plications, as 

 already pointed out, the greatest emphasis will appear parallel to the canal or 

 on the edge of the pillar, where a single strong plait will first be formed, with 

 the possibility of others less well defined and smaller behind it. The theory 

 is in absolute accordance with the facts observed in CryptorJiytis, Piestochilus, 

 Volutomorpha, Liopeplum and Volutilitlies. In the latter, which in its typical 

 form belongs to the earliest Tertiaries, we have a strong anterior plait and a 

 few less definite, closely adjacent, posterior ridges. It is evident, however, 

 that a few elevated thin plaits would afford a larger area of attachment and a 

 better fulcrum for the adductor muscle than a mass of obscure, little-elevated 

 ridges. To natural selection, therefore, I would ascribe the speedy develop- 

 ment of the former type and the rather rapid decrease in the number of species 

 with the latter type of plications, so that at present only one or two living 

 species are known, while in the early Tertiaries it was the dominant type. I 

 should state, perhaps, that those plaits which follow the pillar into the spire 

 are alone under consideration, those accessory ridges about the aperture being, 

 like the lirse of the outer lip, of a very subordinate importance. With the in- 

 crease in size and elevation is associated a reduction in number of plications 

 until what may be supposed to be the most advantageous number is arrived 

 at. In such species as Scaphella magellanica Sby., where the primary plaits 

 are not very prominent, the tendency to develop feeble accessory ridges often 

 asserts itself 



