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 diificult 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 

 4^ inches in length, in company with Fh. Candida. 



Ph. dactylus is in Mr. Smith's * List of Clyde Fossils.' 



Pholadidea, Leach, 1819. 



Pholas. Turton. 



Martesia. Leach, 1818. Blainv. 1824. 

 JoTJANNETiA? Besmoul. 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 Fholades 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., Fholadojjsis, Conrad, and Tnomjihalia, 

 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 Fara- 

 pkolas. 



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 



