574 PROCEEDINGS OF THE ACADEMY OF [Oct., 



writer h^s been able to learn, F. coronatmn has not been found in 

 these earlier beds. Those favoring purely stratigraphic methods 

 will probably argue that, in any phylogenetic scheme, F. coronatum 

 should be represented as derived from F. rugosum. This, however, 

 would require an explanation of the absence of a keeled stage in 

 F. coronatum and the acceptance of a theory that ontogenetic stages 

 do sometimes go through a process the reverse of acceleration. 

 The author inclines to the morphologic method in this case. It is 

 believed that the derivation of F. rugosum from F. coronatum or 

 something very close to it is amply justified, in spite of the fact that 

 the latter form has not yet been proved to extend farther back than 

 the St. Mary's Miocene. 



In passing it may be said that the F. jjyrumASke forms from the 

 Duplin Miocene have been omitted purposely from the scheme, 

 because of the suspicion, as yet unverified, that most museum sets 

 are made up of specimens from more than one horizon in the Duplin 

 beds. 



The CORONATUM-INCILE SEQUENCE. 



The second morphologic sequence which commands our attention 

 starts, as before, with F. coronatum, but ends with the late Miocene 

 F. incile Conrad. ^^ The gradations in this sequence, though not as 

 complete as in the F. coronatum-F . pyrum sequence, are perhaps no 

 less striking. Its members comprise F. coronatum Conrad (St. 

 Mary's Miocene), F. rugosum Conrad (St. Mary's Miocene), F. 

 canaliculatum (L.) (Pleistocene and Recent), and F. incile Conrad 

 (Burwell Bay Miocene). These are shown in Table II. As before, 

 the sequence consists (1) in the progressively earlier appearance of 

 the canal in passing from F. coronatum to F. incile; (2) the progress- 

 ive shortening of the noded stage through compression of the stage 

 and acceleration of its later part; (3) the introduction and progressive 

 acceleration of the keeled stage and its final compression in the 

 terminal meml^er, and (4) the introduction of a final rounded stage 

 in F. incile. 



In the succession of its ontogenetic stages, F. incile parallels F. 

 pyrum, but in other respects it is quite different. In F. pyrum the 

 tendency is for a shortening of the spire and a lengthening of the 

 anterior canal. For F. incile, on the other hand, the reverse is true; 



""Descriptions of Miocene Shells of the Atlantic Slope," T. A. Com-ad, 

 Am. J. Conch., p. G-4, pi. G, fig. 2, 1868. 

 See also Aju. J. Sci., vol. XXIIl, p. 343. 



