XXIV STRUCTURE OF THE SHELL. 



SHAPE AND STRUCTURE OP THE SHELL. 



The shape of the shell has been varied much less in Acha- 

 tinellidce than in Amastridce. Except in Newcombia, where 

 the spiral has been conspicuously lengthened, the ovate or 

 ovate-conic contour has been modified only in minor features. 

 In sculpture the diversity is of degree. Nearly all species 

 are polished, showing weak growth-lines and minute spirals, 

 often descending obliquely, and frequently faint or absent on 

 the later whorls. The later whorls of the embryonic shell are 

 always minutely striate spirally. In Partulina the spiral 

 stride are often rather well developed ; and in Newcombia they 

 become prominent. 



The axis of the shell is somewhat sinuous, and in the last 

 whorl it bears a spiral lamella except in Newcombia, Perdi- 

 cella and Baldwinia, where it is more or less degenerate. 

 This lamella is progressively absorbed as the shell grows. 

 In the embryonic stage it is represented only by a sinuation 

 of the columella. In both Achatinellidce and Amastridce the 

 forms lacking a columellar lamella are obviously not directly 

 related, being terminal members of several evolutionary series. 

 Thus Perdicella, Newcombia and Baldwinia have more direct 

 affinity to Partulina than to one another. That the ancestral 

 stock of Achatinellidcz, Amastridce and Tornatellinidce had a 

 columellar lamella seems a fair inference. 



Professor Hyatt's idea that Partulina dubia is a representa- 

 tive of the ancestral stock of Partulina seems to me inad- 

 missible, as its weak columella is far more likely to be a recent 

 degenerative character than a primitive heritage, and it has 

 no other feature more primitive than other Partulinas. 



The direction of coil of the shell and the dextral or sinistral 

 disposition of the organs are unusually variable in Achatinel- 

 lidcc ; far less so in Amastridcz, where sinistrality is unusual, 

 and in all cases a specific or group character. Many species 

 of AchatinellidcE are invariably sinistral. In Newcombia all 

 are sinistral. Other species are as invariably dextral. In 

 such species a reversed specimen is as rare as among Ameri- 

 can land snails. A few species of Partulina and many of 

 Achatinella are indifferently dextral and sinistral, either in 



