646 



THE AMERICAN NATURALIST 



[Vol. XLI 



while the one just above the suture also becomes very pronounced. 

 Sometimes this latter is prevented from taking part in the orna- 

 mentation of the shell by the close coiling of the whorls, the suc- 

 ceeding whorls crossing this spiral. (^Martini Chemnitz, I 24, pi. 

 37, fig. 3-3a). Differentially accelerated specimens may have the 

 flange pushed back into the ribbed portion; i. e. the flange appears 

 before the ribs disappear. Then the flange is broken up into 

 blunt vertically flattened spines which unite into a keel as soon as 

 the ribs disappear entirely. 



We have thus in the Melanias, a group of highly accelerated 

 gastropods in which the spines, — a specialized feature, appear- 

 ing late in the phylogeny of most gastropods — ■ have become a 

 dominant character, appearing before the ribs have disappeared. 

 Many phylogerontic members of this group, forming terminals of 

 genetic series, retain their ornamentation only in the young, the 

 adults becoming smooth. In several lines extreme accentuation of 

 certain characters at the expense of others has resulted in grotesque 

 forms. All the characters, however, appear and disappear in a 

 regular progressive manner both in ontogeny or individual devel- 

 opment, and in phylogeny or the development of the genetic series. 

 The Melanias therefore constitute an excellent group from which 

 illustrations of ortho-ontogenesis and ortho-phylogenesis may be 

 obtained. 



