110 PHOLADID.E. 



more persistent on the posterior side : margins acutely an- 

 gular or beaked at the anterior end, with a wide oval gape 

 towards the front, whence there is a regular slope above and 

 below to the posterior end, which is broad and rounded, with 

 sharp edges and a moderate gape ; dorsal margin longer than 

 in P. Candida, concave and sculptured like the rest of that 

 side : beaks placed at a distance of about |-ths from the 

 anterior end : hinge-line flexuous : hinge-plate extremely 

 broad, folded over the umbonal area but not adhering to any 

 part of it; the centre is marked as in the last species, and 

 furnished with a thick knob or tubercle, which apparently 

 serves by its intervention to prevent the valves from being 

 squeezed too closely together; the crown of this tubercle is 

 consequently more or less worn by continual pressure, and it 

 is connected with the dorsal posterior margin by a sharp ridge, 

 so as to give it additional strength : apophyses of moderate 

 breadth, not much curved, and nearly flat : dorsal shield some- 

 what curved, and lanceolate with the point outwards ; it has a 

 small boss close to the broader end, which is bent inward ; 

 there is a slight depression down the middle, and the lines of 

 growth are distinct, diagonally arranged, and numerous : in- 

 side polished and occasionally iridescent, usually showing the 

 external sculpture, and having the edges notched on the an- 

 terior side. L. 0*8. B. 1*85. 



Var. quadrangida. Shell smaller and more contracted 

 at each end, with closer and finer sculpture. 



Monstr. tubercidata. Shell divided into two nearly equal 

 parts by a longitudinal irregular furrow. P. tuberculata, Turt. 

 Conch. Dith. p. 5, t. 1. f. 7, 8. 



Habitat : New-red sandstone, marl, clay, and sub- 

 marine peat, at Guernsey and on the southern coasts of 

 England; Oxwich Bay near Swansea (J. G. J.) ; Aber- 

 gelly, Denbighshire (Pennant) ; Dublin Bay (Warren) ; 

 near Belfast (Hyndman) ; St. Cyrus, Kincardineshire 

 (Brown). The North Welsh and Scotch localities are 

 doubtful ; because Pennant's shell was probably the 

 young of P. crispata, and the single specimen said to 

 have been found at St. Cyrus may have been from 

 ballast. The variety is from indurated clay, and the 



