HIPPURITIDA. 205 
gills into the exhalent channel. The smallness of the space for 
the branchiz may have been compensated by deep plication of 
those organs, as in Chama and Tridacna. 
HIPPURITES (restricted). Hinge-rib well-developed. H. cornu- 
vacctnum, Bronn. 
D’ORBIGNYA, Woodward, 1862. ‘‘ No ligamental inflection of 
the outer shell.” Doubtfully distinct. Fossil, 4 sp. Middle 
Chalk; Europe. 4. bioculatus, Lam. 
BARRETTIA, Woodward, 1862. Dedicated to Mr. Lucas Barrett, 
late Director of the Geological Survey of the West Indies. 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. Type, 
B. monilifera, Woodward. ‘ Huppurite limestone.” Jamaica. 
This is a doubtful group; its pertinence to the genus, and even 
to mollusea, has been questioned. 
PIRONHA, Meneghini, 1868. Shell strongly ribbed; the hinge- 
lamina short and thick. H. organisans, Mont. 
RapvrouirEs, Lamarck, 1801. 
Etym.— Radius, a ray. 
Distr.—Fossil, 42 sp. Neocomian—Chalk; Texas, Britain, 
France, Bohemia, Saxony, Portugal, Algeria, Egypt. 2. alata, 
d’Orb. (exviii, 31). BR. mamillaris, Math. (cxviii, 32-35). BR. 
Heeninghausit (cxviii, 36, 37). 
Shell inversely conical, biconic, or cylindrical; valves dis- 
similar in structure ; internal margins smooth or finely striated, 
simple, continuous ; ligamental inflection very narrow, dividing 
the deep and rugose cartilage-pits ; lower valve with a thick outer 
layer, often foliaceous; its cavity deep and straight, with two 
dental sockets and lateral muscular impressions; upper valve 
flat or conical, with a central umbo; outer layer thin, radiated ; 
umbonal cavity inclined towards the ligament; teeth angular, 
striated, supporting curved and subequal muscular processes. 
The upper valve of &. fleuriausus has an 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 cavity of the lower valve is partitioned off by 
very delicate funnel-shaped laminew. Specimens frequently 
eceur 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, 
