ORBICULA. 
Puate I, 
Genus ORBICULA, Sowerdy. 
Testa suborbicularis, inequivalvis, valvd superiore nunc co- 
noided nunc depressd, testaced vel corned, interdum 
levigatd, sepe radiatim striata, posticé tumidd, vertice 
subcentrali, posticé inclinato; valvd inferiore tenut, 
corned, vel concentricé vel radiatim striata, disco 
postico, subguadrato-cordato, pediculo adhesionis fis- 
surato. Cardo aut ligamentum nullum. 
Shell somewhat orbicular, inequivalve, upper valve some- 
times conoid, sometimes depressed, testaceous or 
horny, sometimes smooth, often radiately striated, 
posteriorly swollen, vertex subcentral, inclined pos- 
teriorly ; lower valve thin, horny, either concentri- 
cally or radiately striated, with a posterior disk slit 
for the passage of the pedicle of adhesion. No hinge 
or ligament. 
A genus with the name Ordicula was founded by La- 
marck for the reception of a Northern brachiopod, Patella 
anomala, Miller, which proved to be dnomia craniolaris, 
pars, Linneus, and the type of Retzius’s genus Crania. 
A Mediterranean Crania (Anomia turbinata, Poli,) was 
considered synonymous with it, but it has been shown to 
be distinct. On meeting with the first-discovered speci- 
men of the group before us, Lamarck created a genus Dis- 
cina, while Mr. Sowerby confounded it with the Northen 
Crania, of which Lamarck had made an Orbicula. Des- 
hayes, Broderip, and Sowerby, out of all this confusion, 
adopted Ordicula for the brachiopods under consideration, 
and it appears to me to be unnecessary to set aside their 
decision. 
The shell of Ordccula differs from that of Crania in the 
very important particular of having the under valve thin 
and horny, furnished with a disk in which is a slit for the 
passage of a pedicle of attachment. The disk is subcentral, 
more or less inclined to posterior, and corresponds in po- 
sition with the vertex of the upper valve. In some species 
the vertex is posterior, and the disk is also posterior; in 
others the vertex is a little posterior to central, and the 
disk is also a little posterior to central. That is the natu- 
ral symmetrical growth of the shell on a plane surface. 
But ifthe animal adheres to a declivitous surface, the disk 
and vertex, which on a plane surface would be subcentral, 
press more posteriorly ; and if the surface be hollow, then 
the under valve is more concave, and the vertex and disk 
more central. 
Seven species of Orbicula are now known. We have 
none in the European seas. The original species, found 
in ballast, O. ostreoides, is supposed to be a native of 
North or West Africa, and there is a small species, O. 
stella, in the Eastern Seas. The rest are inhabitants of 
the New World, where Crania is unknown. O. Antillarum 
is the analogue of O. stella in the West Indies, and 0. 
Cumingii, an allied form of more solid growth, ranges 
from Peru to Panama, Guatemala, and Mazatlan. But the 
most striking type is that represented by three species on 
the coast of Peru, P. lamellosa, levis, and tenuis, of the 
last of which Mr. Cuming possesses specimens, strange to 
say, from South Australia. 
Species 1. (Fig. 1 a, 6, Mus. Cuming.) 
ORBICULA STELLA. Ord. testd orbiculari, convexiusculd, 
subtilissimé radiatim striatd, striis minute concentricé 
clathratis, vertice subcentrali, levigatd, interdum car- 
neo tinctd et radiata. 
THE sTAR OrBicuLA. Shell orbicular, rather convex, 
very finely radiately striated, strie minutely concen- 
trically latticed, vertex nearly central, smooth, some- 
times tinged and rayed with flesh-colour. 
Discina stella, Gould, Exped. Shells, Pro. Bost. Soc. 1846. 
Otia Conch. p. 120. 
Hab. China Sea, Wilkes; Singapore and Philippine Islands, 
Cuming. 
This species has a wide distribution in the Eastern Seas. 
On comparing authentic specimens received from Dr. 
Gould, one of which is given at Fig. 1 4, collected in the 
China Sea by Wilkes’ Exploring Expedition, I find them 
identical with specimens collected by Mr. Cuming, attached 
to fragments of Pullastra, Pinna, and Maileus, at Singa- 
pore and at the Philippine Islands. The sculpture varies 
in strength; in young specimens, as stated by Dr. Gould, 
the radiating strie are scarcely developed; in older speci- 
mens, and especially specimens that have had to contend 
with irregularities in their place of attachment, the sculp- 
ture has a minutely latticed character, like the grain of a 
thimble. The under valve is thinly membranaceous or 
thicker, concave or convex, according to circumstances 
of habitation ; and the position of the slit obviously varies: 
with the position of the vertex in the opposite valve. On 
a flat place of attachment the shell is symmetrically orbi- 
June, 1862. 
