CIRRIPEDIA. 



501 



f- 



the operculum of the Balanoids. The next most important are the 

 "terga" (fig. 358, a), which protect the dorso-lateral surface. A 

 pair of scuta and a pair of terga are present, and these are the 

 largest of all the valves. The " carina " and " rostrum " are placed 

 along the edges of the ca- 

 pitulum, the former being 

 much the most important ; 

 and there may be a " sub- 

 carina " and " sub-rostrum." 

 The remaining valves, with 

 the carina and rostrum, cor- 

 respond with the proper 

 shell of the Balanoids ; but 

 they are often wanting or 

 rudimentary, and they re- 

 quire no further considera- 

 tion here. 



As regards the distribu- 

 tion of the Lepadoid Cirri- 

 pedes in time, the oldest 

 known types belong to the 

 genus Turrilepas i K = Plum- 

 ulites), several forms of 

 which have been detected 

 in the Ordovician, Silurian, 

 and Devonian formations. 

 In this singular genus (fig. 

 359, a) are included elon- 

 gated bodies covered with 

 from four to six intersect- 

 ing rows of calcareous plates. The plates are triangular in form, 

 with a curved lower margin, and marked with transverse striae ; 

 those of the median series being keeled. When detached and 

 occurring in an isolated condition (fig. 359, a), these plates are 

 not unlike the shells of certain Pteropods, while they may be, and 

 have been, mistaken for the plates of Chitons. They are also not 

 very unlike the plates of some Cystideans (Ateleocystites). By Bar- 

 rande, Turrilepas was regarded as the capitulum of a Lepadoid, in 

 which there was either no peduncle or a short one ; but Dr Henry 

 Woodward is probably correct in the view that the fossils really 

 represent a scaly peduncle similar to that of the genus Loricula, 

 and that the capitulum is still unknown. On the other hand, the 

 Strobilepis of the Devonian rocks of North America seems to be 

 really founded upon the capitulum of a Lepadoid. Allied to 

 Turrilepas is the Silurian genus A?iatifopsis, of which only the 



Fig. 358. — Capitulum of a Pedunculated Cirripede. 

 a, Tergum ; b, Scutum ; c, Carina ; d, Upper latus ; 

 e, Carino-latus ; f, Rostrum ; g, Sub-rostrum ; k, Ros- 

 tral latus ; z, Infra-median latus ; k, Sub-carina. (After 

 Darwin.) 



