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THE AMERICAN NATURALIST [Vol. XLI 



which we can seize for the purpose of determining the hne of prog- 

 ress. The earliest types were undoubtedly marine giving rise on 

 the one hand to the fresh water Melanias and on the other hand to 

 a number of marine descendants. No exhaustive discussion is 

 contemplated here, this and the tracing of the various genetic lines 

 beuig reserved for a future work. Only some of the more salient 

 features developed during the study of this group of shells will be 

 mentioned here as illustration of the principles discussed. 



The Eocene Melanoides praecessa and M. inquinata of the Paris 

 basin may be taken as a starting point, though they already present 

 characters of considerable complexity both showing a marked series 

 of changes. In the former the earliest stages observed show 

 rather flatly rounded whorls with distinct, narrow, rounded ribs 

 which are concave forward (i. e., towards the aperture). These 

 ribs are cancellated by revolving spirals which however are faint 

 in the interspaces, but form pronounced nodules on the ribs. In 

 some specimens the earliest ribs appear to be free from these 

 tubercles, and the mode of appearance seems to indicate that the 

 ribs precede the spirals. This is the character of the early stages 

 of the type specimen figured and described by Deshayes (Des., 

 An. sans, vert., II, p. 452, pi. 23, figs. 31-32) these stages being 

 free from spirals. This character recalls the adult of Pseudome- 

 lania {Chemnitzia) undulata d'Orb., Ch. carmensis d'Orb., Ch. 

 ccrrvalittna and Ch. periniana d'Orb., from the Middle Lias and Ch. 

 rhodani of the Upper Lias of France.' In later Jurassic strata of 

 France occur many Pseudomelanias, which show no ribs in the adult. 

 Some of these may however possess them in the young, as has 

 actually been found in the case of P. nerei d'Orb. of the Bathonien. 

 Such occurrences suggest that the smooth Pseudomelanias are 

 derived from the ribbed ones and hence in so far as they have 

 lost this character are phylogerontic. 



In Eocene Melanoides praecessa var. spiralis var. nov. from 

 Noyon (C. U. coll. 30041, fig. 10) the spirals become gradually 

 strengthened, until they are well marked. The uppermost or pos- 

 terior spiral becomes strongest; and gradually the space above it 

 develops into a concave shoulder free from ribs. On this shoulder- 



» D'Orbigny. Paleontologie Francaise. 



