558 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



PECTINI BRANCHIAT A. 



CERITHIIDiE. 

 Genus CERITHIDEA, Swainson. 



Synon. — Cerithium (sp.), of authors; not of Brug. as restricted by Lamarck and others. 



Cerithidca, Swainson (1840), Malac, 203 and 324.— Gray (1847), Zool. Proceed., 154.— H. and A. 



Adams (1854), Genera Recent Moll., I, 292.— Chenu (1859), Man. Conch., 1, 286 (as a 



subgenus under Cerithium). 

 Pircnella, Gray (1847), Zool. Proceed, 154.— H. and A. Adams (1854), Genera Recent Moll., 1, 293 



(as a subgenus tinder Cerithidca): — Chenu (1859), Man. Conch., I, 285 (as a subgenus 



under Ceiithium). 



Etym. — Cerithium. 



Examp. — C, decollate/, Linnaeus. 



Shell elongate-conical, many-whorled, more or less decollated at the 

 apex ; volutions regularly or irregularly costated, or granular ; aperture 

 rounded, slightly emarginated, or with a short anterior canal; outer lip with 

 an expanded and thickened margin, or thin and sinuous. 



As proposed by Swainson, this genus was made to include species 

 belonging to several distinct groups ; but I adopt it here as used by H. and 

 A. Adams, who admit the following two subgenera under it : 



1. cerithidea, Swainson (typical). 



Shell with volutions longitudinally ribbed ; aperture slightly 

 notched or emarginate anteriorly ; outer lip with a dilated thickened 

 margin. — (Example as already stated.) 



2. pirenella, Gray. 



Shell irregularly ribbed, or granular; aperture with a sbort 

 anterior canal ; outer lip thin and sinuate ; inner lip simple. — (Cerithi- 

 •um mamillatum, Philippi.) 



The species of this genus are capable of living either in brackish- or 

 fresh-waters, or on land near the same, being frequently found crawling 

 about mangrove-swainps in southern countries. They live so much out of 

 water that they have even been supposed to he true land-shells. During 

 continued dry seasons, they suspend themselves to branches of trees by a 

 kind of threads formed by glutinous matter secreted for that purpose, the 

 moutb of the shell being at the same time closed by the operculum. 



This genus is known to occur in the Eocene of tbe Old World, and 

 possibly may date as far back as the closing part of the Cretaceous period in 

 the Upper Missouri and Rocky Mountain regions of this country. 



