BIVALVIA. 297 
fragment of a specimen having been there found by myself. It seems to have become 
much more abundant in the succeeding Period, as it is by no means rare at Walton 
Naze, but difficult to procure, the shells generally separating at the suture. It has also 
been found in the Drift Beds in Ireland, and ranges, in the living state, on the N. E. 
Coast of America, as far as South Carolina. The characters of this species are well 
marked, so as not easily to be confounded with any other. It is found in a very modern 
Tertiary Deposit at Bracklesham, where specimens have been obtained measuring 
43 inches in length, in company with PA. candida. 
Ph, dactylus is in Mr. Smith’s ‘ List of Clyde Fossils.’ 
PHouapDIDEA, Leach, 1819. 
Puouas. Turton. 
Marresta. Leach, 1818. Blainv. 1824. 
JOUANNETIA? Desmoul. 1828. 
PHOLIDEA. Swains. 1835. 
Generic Character. Shell ovate or oblong, equivalve, inequilateral, externally 
rough or imbricated. Anterior extremity open in the young, but closed in the adult 
shell. Posterior extremity truncated and gaping, furnished with a coriaceous or 
testaceous cup when full grown. 
Animal club-shaped; mantle closed, except a small opening in front for the passage 
of a truncated, sucker-shaped foot. Siphonal tube long, terminating in a disk, sur- 
rounded with cirrhi; terminal openings also fringed. 
This genus has been founded upon a species of one of the rock-boring molluscs, 
whose great peculiarity is, that when it has arrived at the full stage of existence, it 
closes the previously large opening in front with a thin calcareous covering ; and at its 
posterior termination there is added a small testaceous cup at the base of the siphons. 
If this be entitled to generic distinction it must rest its claim upon the latter character, 
as many of the P/olades have a large pedal opening in their young state for the active 
employment of that organ, possessing the same habits as the animal of this genus, in 
closing the aperture when full grown by a calcified membrane. Some other species 
also endowed with this habit, though not strictly according with the diagnosis of this 
genus, appear to be very closely related, viz., Pholadopsis, Conrad, and Triomphalia, 
Sowerby, but the valves are of unequal magnitude. Other species, possessing two 
radiating furrows, have been proposed for a genus by Conrad, under the name Para- 
pholas. 
The prolongation of the shell at the posterior side appears to be the commencement 
of what, in proximate genera, become a lengthened calcareous tube for the protection 
of the elongated siphons, as pointed out by Messrs. Forbes and Hanley, when con- 
39 
