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THE AMERICAN NATURALIST 



[^-()L. XLI 



strengthening of the ribs and spirals until in the spiral whorl the 

 ribs and spirals gradually disappear. So far as shown by the 

 specimens examined, which all lack the apical whorls, the ribs are 

 absent from the early carinate whorls. It is possible that the cari- 

 nate whorls are preceded by rounded ribbed whorls but of this 

 we have at present no evidence. Should this eventually prove not 

 to be the case, the explanation of these characters must be found 

 in the early acceleration of the carina, which appeared on the 

 smooth whorls before the ribs had appeared these latter not coming 

 in until the time of disappearance of the carina. In M. sirigosa 

 Lea from the same locality, ribs seem to be wanting altogether, 

 the early angular whorls passing by disappearance of the keel 

 and spirals into a smooth adult form. Other species having 

 smooth adult, and ribbed and spiralled youthful form are M. rusiica 

 Moussen (Martini diemnitz pi. 17; 2 a, b), M. palimpsrshs Reeve 

 (.Martini Chemnitz pi. 17; 3;, and J/. ha.siuJa Lea (Martini Chem- 

 nitz pi. IG; 3, a-d). 



Claviger subaurittis Brot represents the more ])rinutive condition 

 of that genus, in which the whorls are ribbed, the single row of 

 tubercles being developed only on the last whorl (see Martini- 

 Chemnitz I 24, pi. 36, fig. 11a). 



The extremely spinose types of Melanias such as Claviger 

 hyrmiensis Gray (Martini Chemnitz pi. 36; lOa-c), and C. auritus 

 Miiller (Martini Chemnitz pi. 36; 7a-c), represent accentuations 

 of one character at the expense of the others; — a one-sided 

 acceleration. In the latter species the nepionic whorls show ribs 

 and spirals the central of which become fused with two adjoining 

 ones. This fused series quickly becomes accentuated in a pro- 

 nounced tubercle, which increases in size and becomes variously 

 modified while the remaining portion of the shell becomes smooth. 

 At first the tubercles are near the middle of the whorl but -radu- 

 ally as tliey increase, the rehitive amount of enibracin,- increases 

 until the lui.ercles of the aduit whorl lie just above the siuure. 



PofamiJt.s' jL-atus of (;an.l)ia. The nei)ionic and early neanic 

 stages are identical with the adult of P. granidatus (Brug.) 



