AVICULIDA. 279 
Hemispherical pearls have been found developed from their 
inner surface, and spherical pearls of the same prismatic-cellular 
structure occur detached, in the chalk.—WerHERELL. ‘The Ino- 
cerami of the gault are nacreous. 
CATILLUS, Brongniart, 1822. (Haploscapha, Conr.) Shell 
flattened or ventricose, elongated, cordiform or suborbicular, 
subequivalve, inequilateral; hinge-line nearly straight, its margin 
with a short series of small cavities, gradually enlarging ; shell 
fibrous. J. Lamarckii, Brongn. (cxxx, 56). Cretaceous. 
ACTINOCERAMUS, Meek, 1864. Proposed for a small group of 
Inocerami of the type of J. sulcatus, Park. (exxx,57). They have 
a rather short and sometimes oblique hinge-line, the left valve 
is often slightly more globose than the right, and both are dis- 
tinguished from other similar forms by the presence of radiating 
ribs. The fibrous layer appears to be often thinner in Actino- 
ceramus than in most of the concentrically suicated Inocerami. 
VOLVICERAMUS, Stoliczka. Type, L. cnvolutus, Sow. (exxx,58 , 
which has the left valve strongly involute like a nautilus or 
somewhat resembling a Gryphiea, while the right valve is very 
much smaller, flattened, slightly tumid at the apex, resembling 
an operculum ; the hinge- line is curved, conformed to the ovately 
rounded aperture of the left valve, thickened and provided with 
numerous ligamental pits, as in other typical species of the 
genus. 
Anopma, Eichwald, 1861. 
Distr.—3 sp. Cretaceous; Russia. 
Equivalve, inequilateral, elongated, with the shorter anterior 
part narrower, beaks close together, with a deep circumscribed 
lunule in front of them; ligament situated in a number of 
roundish pits in the straight cardinal margin, which has a rib- 
like tooth in the left valve below the beak, extending anteriorly 
for a short distance. Principally differs from Inoceramus by 
the presence of a deep lunule and by the internal hinge-rib (in 
the left-valve). 
GERVILLIA, Defrance. 
Etym.—Dedicated to M. Gerville, a French naturalist. 
Distr.—Fossil, 37 sp. Carb.—Chalk; Europe. G. anceps, 
Desh. (exxxii, 90). 
Shell like Avicula; elongated; anterior ear small, posterior 
wing-like; area long and flat, cartilage-pits several, wide apart ; 
hinge-teeth obscure, diverging posteriorly. 
BAKEWELLIA, King. Shell small, inequivalve, caitilage-pits 
2-5 ; hinge with anterior and posterior teeth; anterior muscular 
impression and pallial line ibe eos Fossil, 5 sp. Permian; 
Britain, Germany, Russia, U.S. G.antiqua, Miinst. (cxxx, 59; 60). 
