BIVALVIA. 95 
pression of which may be generally seen, one on each side and above the adductors, within 
the extended dental margin. 
Although the mantle is generally open all round, the animal is capable of contracting 
or uniting the edges on one side, so as to form two openings, one for the incoming 
current and the other for the outgoing, being the commencement of the true siphons. 
The known recent species of the genus are about sixty or seventy, and perhaps a similar 
number in the fossil state ; these last are very difficult of determination, from the generally 
slight deviations in the form of the shell, the normal condition being nearly lenticular, the 
specific distinctions depending principally upon the sculpture of the surface or dental 
characters ; but these teeth are very fallacious, as some are obliterated by age. 
The peculiar form of these shells are favorable to their preservation, offering, 
as they 
do, a protection from mutilation, and specimens are often in high perfection. 
The genus, in the recent state, has a wide geographical extension, but the species are 
somewhat restricted in their range; they are principally inhabitants of warmer regions, 
although P. glycimeris is living in the British seas, and P. sepéentrionalis in those of the 
north-west coast of America. A species found in the Eocene deposits of North America 
is said to be identical with one of our own fossils of the same age. It is most difficult, as 
before observed, to determine identity in shells of this genus; but, assuming it to be as 
so stated (which I much doubt), we may, I think, fairly place this species in the same 
category as Terebratulina caput-serpentis, Kellia suborbicularis, and many other living 
molluscs, whose localities at the present day are separated by apparently impassable 
barriers. We are not able now to trace these animals, whose localities are so unconnected, 
to what may be assumed as a common ancestry for each species thus identified. Whether 
these apparently identical forms are descendants of ancestors belonging to the same species 
once living together in close geographical contiguity, or whether they are forms having a 
distinct origin, but presenting no difference by which the malacologist can separate them 
from the typical species, we have at present not the materials to determine. 
1. Pecruncunus Brevirostris, J. Sowerby. ‘Tab. XVI, fig. 8. 
PECTUNCULUS BREVIROSTRIS. J. Sow. Min. Conch., t. 472, fig. 1, 1524. 
— — Id. in Dixon’s Geol. of Suss., p. 225, t. 14, fig. 32, 1850. 
— BREVIROSTRUM. Morris. Catal. Brit. Fos., p. 219, 1854. 
— PULVINATUS. Mantell. Geol. of Suss., p. 273, 1822. 
PectuncuLus. Smith. Strata Identif., t. 11, fig. 3, 1816. 
Spec. Char. P. testdé suborbiculart vel obovatd, convexo-lenticulari viv inequilaterali, 
sub-symmetricd ; radiatim obsolete costellatad ; concentricé striatd ; umbonibus brevibus de- 
pressis ; ared connexis magna, area dentali arcuatd ; dentibus paucis magnis ; marginibus 
crenulatis. 
