502 



CRUSTACEA. 



detached valves are known, these being somewhat quadrilateral in 

 shape, and having the lower part of the base marked out into one 

 or two horizontal segments, which are more or less separated from 

 the body of the valve. In the Carboniferous and Permian rocks 

 no Lepadoids are known, but the genus Pollicipes is represented 

 in the later Triassic (Rhsetic deposits). Other forms of Pollicipes 

 are Jurassic, but the genus attained its maximum of development 

 in the Cretaceous period, at which time the group of the Lepadoids 

 seems to have reached its culminating point. Tertiary species of 

 Pollicipes are also known, and the genus survives at the present 



Fig- 359-— A, Turrilepas (Plumulites) IVrig/itiz'—SUurian. (After Woodward.) a, A plate of 

 the same magnified, a, Loricula pulchella — Chalk. (After Darwin.) 



day. Some Jurassic types formerly ascribed to Pollicipes have been 

 separated by Zittel to form the genus Archceolepas. The genus 

 Loricula is wholly Cretaceous, and comprises forms with a small 

 capitulum and a short, plated peduncle (fig. 359, b), the organism 

 being attached by one side to the shells of Ammonites. The exten- 

 sive genus Scalpellum has numerous living types, and also about 

 thirty fossil representatives, of which the oldest appears in the Cre- 

 taceous rocks. On the other hand, the recent genus Lepas (Ana- 

 tifd) is not certainly known to occur in the fossil condition, even 

 in the late Tertiary or Quaternary deposits, though the allied 

 Poecilasma is doubtfully represented in the newer Tertiaries. 



