CHILONOPSIS. 173 



C. aurisvulpina and most of the other species existed, and 

 there is every reason to believe that the snail fauna was wholly 

 unaffected by deficient or irregular water supply. The shells 

 had not the coloration of arid-country snails. C. anrisvidpiitx 

 is a markedly phylogerontic form, and was probably nearing 

 extinction when the catastrophy was precipitated by the de- 

 foresting of the island. The sculptural peculiarities of the 

 others are easily paralleled in forms inhabiting notably humid 

 regions such as St. Thome, Cameroons, etc. 



St. Helena exhibits two common characteristics of almost 

 all old island-faunas: (1) a very small number of original 

 generic types, in this case about six, some one or two of 

 which have been greatly modified to fill stations commonly 

 occupied by species of several genera. Cf. Pcecilozonites in 

 Bermuda; Mandarina and Hirasea in the Bonin Is.; Acha- 

 tiucUida- in the Hawaiian group; (2) accelerated aging of 

 the genera so modified, resulting in unusually large forms 

 variously calloused or irregularly coiled, being often unduly 

 elevated or depressed for their genera, stigmata of old age 

 of the group. This results in an unusual proportion of ex- 

 tinct species, especially if the equilibrium of the fauna be in 

 any way disturbed ; and the forms w r hich drop out are as a 

 rule, those with phylogerontic characters. Cf. in Bermuda, 

 Pcecilozonites nelsoni, cupula, zonata; in the Bonins, Man- 

 darina ruscheribergeriana, pallasiana; in the Madeiras, Geo- 

 miira delphinida, bowdichiana, Leptaxis loivei, etc. In New 

 Caledonia the most ponderous and heavily calloused Placos- 

 tyles. The list could be greatly extended. 



It seems therefore that the characteristics of the St. 

 Helena snail fauna are those of an ancient, isolated fauna, 

 rather than traceable to aridity of climate. 



NOMENCLATURE. The name I'achyotus has been used for 

 C. aurisvulpina by most authors, in either a generic or sub- 

 generic sense. That name was proposed by Beck in 1837 for 

 a series of Brazilian forms and the species B. aurisrulpina, 

 no type being selected. Herrmannsen, in September, 1847, 

 selected the Brazilian species Bidimus melanostomus as type, 

 thus making PacJiyotus a synonym of Aio'is Spix. See 



