136 AMASTRA. 



ing the color of the underlying calcareous layer modified by 

 its own tint. 



EMBRYONIC SHELL: The sculpture and, to a less degree, 

 the shape of the embryonic whorls is remarkably varied. In 

 the more primitive forms of Amastrella these whorls are con- 

 vex and almost smooth, showing very faint growth-lines only. 

 In A. tristis the whorls 'are flattened, and either striate or 

 weakly costate longitudinally ; while in most Amastrce of 

 Molokai and Maui the embryonic whorls are flat, strongly 

 costate, and carinate above the suture. Some species, as A. 

 nigra (pi. 43, figs. 1-3) and others, have costate and carinate 

 embryonic whorls in some individuals, while in others they 

 are more convex and striate; the neanic and adult characters 

 being alike. This is apparently a case where the larval form 

 mutates independently of the adult. Similar conditions 

 have been observed by Sturany and others in marine Proso- 

 branchs; but in pulmonate snails no such diversity of the 

 embryo has come under observation except in this genus. 

 Such diversity of embryo with identity of later stages has 

 been observed by both authors in a number of species. It 

 should be remembered in this connection that embryo shells 

 frequently vary in color, quite independently of the later 

 stages, which may be similar in individuals having diversely 

 colored embryonic shells. 



Subgenera and Sections of Amastra. 



a. Shell umbilicate ; embryonic whorls smooth. Cyclamastra. 

 a 1 . Shell perforate, rimate or imperforate (or in some Amas- 

 trella' umbilicate, the embryonic whorls striate). 

 &. Last whorl strongly carinate ; shell large and dark, 

 c. Last whorl with a peripheral carina, Kauaia. 

 c 1 . Last whorl with two ridges or carinae. 



Armiella. 

 b 1 . Last whorl not conspicuously carinate. 



c. Embryonic whorls smooth or striate, not car- 

 inate; dextral (except A. thaanumi). 

 d. Cuticle thin, not figured; shell ovate. 



Amastrella. 



