HIPPURITID^. 205 



gills into the exhalent channel. The smallaess of the space for 

 the branchiae ma}^ have been compensated by deep plication of 

 those organs, as in Chama and Tridacna. 



HippuRiTES (restricted). Hinge-rib well developed. H.cornu- 

 iiaccininn, Bronn. 



d'orbignya, Woodward, 1862. "No ligamental inflection of 

 the outer shell." Doubtfully distinct. Fossil, 4 sp. Middle 

 Chalk; Europe. H. bioculatus,lj&xn. 



BARRETTiA, Woodward, 1862. Dedicated to Mr. Lucas Barrett, 

 late Director of the Geological Survey of the West ludies, No 

 " ligamental inflection " as in d'Orbignya, but it presents the 

 further peculiarity of an indefinite number of pallial duplicatures 

 extending all round the margin of the lower valve. T3'pe, 

 B. monilifera, Woodward. " Huppurite limestone." Jamaica. 

 This is a doubtful group ; its pertinence to the genus, and even 

 to mollusca, has been questioned. 



piRONiEA, Meneghini, 1868. Shell strongly ribbed ; the hinge- 

 lamina short and thick. H. organisaiis, Mont. 



Radiolites, Lamarck, 1801. 



Etym. — Radius^ a ray. 



Distr. — Fossil, 42 sp. Neocomian — Chalk ; Texas, Britain, 

 France, Bohemia, Saxony, Portugal, Algeria, Egypt. R. alata, 

 d'Orb. (cxviii, 31). R. mamillaris, Math, (cxviii, 32-35). R. 

 Hceninghausii (cxviii, 36, 37). 



Shell inversely conical, biconic, or cylindrical ; valves dis- 

 similar in structure ; internal margins smooth or fiuel}^ striated, 

 simple, continuous ; ligamental inflection very narrow, dividing 

 the deep and rugose cartilage-pits ; lower valve with a thick outer 

 lajT^er, often foliaceous ; its cavity deep and straight, with two 

 dental sockets aud lateral muscular impressions ; upper valve 

 flat or conical, with a central umbo ; outer laj^er thin, radiated ; 

 umbonal cavity inclined towards the ligament ; teeth angular, 

 striated, supporting carved and svibequal muscular processes. 



The upper valve of R. fieuriausus has au oblique umbo, with 

 a distinct ligamental groove. The foliations of the lower valve 

 are frequently undulated ; they are sometimes as thin as paper, 

 and several inches wide. 



The umbonal cavit}^ of the lower valve is partitioned oflT b}'- 

 very delicate funnel-shaped laminae. Specimens frequently 

 occur in which the outer shell-layer is preserved, whilst the 

 inner is wanting, and the mould (" birostrites ") remains loose 

 in the centre. The interior of the outer shell-layer is deeply 

 grooved with lines of growth, and exhibits a distinct ligamental 

 ridge in each valve. 



In aged examples of R. calceoloides the ligamental inflection 

 is concealed, the cartilage-pits partially filled up and smoothed. 



