124 HELCIONISCUS. 



micaceous rather than hronzed-vietallic luster of the interior of the 

 shell 



I have adopted below a division of the numerous species of 

 Helcioniscus into geographic groups as follows : 



1. Chilian species. 



2. Polynesian and East Indian species. 



3. Japanese and Chinese species. 



4. Species of New Zealand and Australia. 



5. Species of East Africa from the Red Sea to the Cape, and the 

 adjacent islands, 



6. (Species of unknown locality.) 



Without having examined all of the species, a perfectly natural 

 zoological grouping would be impossible. Certain apparently 

 natural groups, however, force themselves upon us. Such are the 

 group of P. sagittata, containing sagittata, tahitensis, ardosicea, amus- 

 siiata, toreuma, nigrolineata. 



The group of P. exarata, containing exarata, argentata, melano- 

 stoma, boninensis, nigrisquamata, stearnsii, grata. 



The group of P. rota, containing rota, capensis, novemradiata, vari- 

 abilis, dunkeri, cernica, eucosmia, garconi, etc. 



The New Zealand and Australian species also form a natural 

 group of equal value with the preceding. 



A number of outlying forms, as well as some described species the 

 shells of which I have not seen, apparently belong to neither of these 

 four assemblages. 



(1). Chilian species. 



The limpets of South America belong mainly to Acmceidce. The 

 Patellidce being represented by Nacella-\- Patinella and by one or 

 two forms of Helcioniscus. Of these, the habitat of P. ardosicea is 

 unquestionable, but that of P. nigrisquamata may still be con- 

 sidered open to revision. 

 H. ARDOSi/EUS Hombron & Jacquinot. PL 32, figs. 63, 64, 65, 66. 



Shell erectly conical, short-oval or nearly circular, the apex nearly 

 central ; slopes straight Sculptured with close radiating striae, 

 which are not granose, every fourth one being somewhat larger. 

 Slate colored or light bluish-olive, darker on the upper part of the 

 cone, the eroded apex usually reddish . 



Interior bluish- white, with silvery and opalescent reflections, the 

 central callus opaque-white tinted more or less with reddish, the 



