Pholadaria.] 



MOLLUSCA. 



115 



confidence assert that no two species of the same genus are 

 more dissimilar and distinct in their essential characters. Inde- 

 pendently of the total want of the ring, and the large frontal 

 gape, the smaller end is rounded, and not as in the last species 

 truncate, and it has no accessorial valves. In addition, also, to 

 the other marks of discrimination, it may be observed, that the 

 valves of the Pholas piapyracea are closed in front by a 6ne 

 elastic membrane, in the middle of which, near the base or 

 rounded extremity, is a small orifice or spiracle, which can be 

 closed at pleasure, and through which water is often ejected." 



4. Pholas sulcata, pi. XLVIII, f. 17, 18. 

 Pholas sulcata, First Ed., pi. 9, f. 17, 18. 



Shell transversely oblong-ovate ; hinge line nearly parallel ; 

 almost equally rounded at both extremities ; a thickening and 

 slight flexure on the superior margin, extending to the anterior 

 side ; from the umbonal region emanate two obliquely longitu- 

 dinal, narrow furrows, and terminate on the basal margin, at 

 which point the shell is longest ; anterior to the sulci, the sur- 

 face is covered with close-set, waved, transverse striae ; and the 

 posterior side with irregular, broken, concentric stria? ; external 

 surface of a yellowish or pale brown hue; inside smooth, white, 

 a curved, flattened, tooth-like process under the umbones, a 

 small denticle at the central point of the umbonal region, and 

 an oblique, longitudinal rib, corresponding to the external 

 furrows. 



This shell was found at Dunbar, and is in the collection of 

 David Falconer, Esq., of Carlowrie. 



5. Pholas tuberculata, pi. XLIX, f. 12, 13. 

 Pholas tuberculata, Turton, Biv., p. 5, pi. 1, f. 7, 8. 



Shell transversely oblong-ovate ; umbonal region nearly cen- 

 tral, provided with a single, oblong, accessorial valve, which is 

 truncate at one end, and reaches to the anterior termination, 

 where the valves meet ; the gape or opening nearly circular ; 

 from the umbones a deep furrow emanates, and terminates on 

 the base ; anterior half of the valves considerably longer than 

 the other, being abruptly lengthened from the mesial furrow ; 

 the superior side abruptly descending, terminating in a blunt 

 beak, the lower side of which is acutely hollowed, producing a 

 strongly arcuated basal line, in this half of the valves, which is 

 covered with close-set, oblique, undulating stria? ; these are 

 lying nearly parallel on the other half of the valves, and gra- 

 dually become obsolete towards the superior edge of them ; 

 this side is rounded, and nearly closed ; inside white, smooth, 

 with a large, rough, wrinkled tubercle, situate on the margin 

 above the teeth, and a longitudinal rib, corresponding to the 

 external groove. 



The longitudinal external furrow will at once distinguish this 

 from P. parva. 



Found at Torbay, but is a rare species. 



Section II. — halves destitute ofa longitudinal furrow. 



6. Pholas dactylus, pi. XLIX, f. 1,2, 3. 



Pholas dactylus, First Ed., pi. 8, f. 1, 2, 3; Pennant, IV, p. 

 76, pi. 39, f- 10; Montagu, p. 20 and 558; Donovan, IV, pi. 

 118; Turton, Biv., p. 8; Fleming, p. 457. 



Shell greatly elongated transversely ; umbonal region placed 

 considerably to one side, reflexed with a series of cells on the 

 external side, and covered with two concentrically striated 

 valves, which are connected longitudinally; on the posterior 



side of these is placed a long, spatuliform, plate-like, accessorial 

 valve, affixed by the connecting membrane; anterior side of 

 the valves with rough, transverse, spinous stria?, terminating in 

 a rounded beak, with a large gape at that side ; posterior side 

 smooth, marked with lines of growth, and terminating obtusely; 

 external surface of an ochre-colour, but varying according to 

 the colour of the substance in which it locates ; inside smooth, 

 white ; below the umbones, a large, flat, curved tooth. 



Found on many of the British and Irish coasts, burrowing in 

 hard clay, marl, and the stumps of old trees. 



This species is sometimes found two inches in length, and 

 seven in breadth. 



7. Pholas Candida, pi. XLVIII, f. 6, 7, 8, 9, 10. 

 Pholas Candida, First Ed., pi. 9, f. 6, 7, 8, 9, 10; Pholas 



candidus, Pennant, IV, p. 76 ; Montagu, p. 24 ; Donovan, IV, 

 pi. 132; Fleming, p. 457 ; Turton, Biv., p. 10. 



Shell thin, fragile, semipellucid, transversely elongated, and 

 white, slightly inclining to yellow, but snow-white in other spe- 

 cimens ; both sides rounded at their extremities; umbonal 

 region covered by an elongated, accessory plate, rounded at the 

 posterior side, and acute at the other extremity, with a hollow- 

 groove in its centre, and obliquely striated on both sides ; 

 umbones placed near the anterior side, which is very short ; 

 the other side long ; surface covered with wide-set, longitu- 

 dinal, and transverse striae, prickly at the anterior side ; hinge 

 reflected, and provided with a small, transverse, slightly bent 

 denticle in one valve; inside smooth, glossy, white, with a 

 slender, arcuated, tooth-like process in both valves, under the 

 umbones. 



Inhabits limestone rocks and hard clay, on many of the 

 coasts of Britain and Ireland. 



8. Pholas parva, pi. XLVIII, f. 11, 12. 



Pholas parva, First Ed., pi. 9, f. 11, 12; Pholas parvus, 

 Pennant, IV, p. 77, pi. 40, f. 13 ; Montagu, p. 22, pi. 1, f. 7, 8; 

 Turton, Biv., p. 9 ; Fleming, p. 457. 



Shell rather strong, transversely oblong-ovate ; rough, and 

 pale yellowish-brown externally ; anterior side narrowed into a 

 produced, somewhat curved beak, its superior margin reflexed, 

 and very open at the extremity ; umbones not quite central, 

 and covered by an elongated, spear-shaped, accessory process, 

 which is rounded at the posterior side, and slightly hollowed 

 in the centre, with arrow-shaped stria? ; margin of the umbo 

 provided with a round, smooth tubercle; external surface, at 

 the anterior side, longitudinally and transversely striated, but 

 smooth on the opposite extremity ; inside smooth, white. 



This is a very local species, but is found plentifully at 

 Torbay. I found one specimen on the beach at St. Cyrus, 

 Kincardinshire. 



9. Pholas striata, pi. XLIX, f. 5, 8. 



Pholas striata, First Ed., pi. 8, f. 5, 8 ; Montagu, p. 26 ; 

 Donovan, IV, pi. 117; Turton, Biv., p. 11; Pholas conoides, 

 Fleming, p. 457. 



Shell transversely conoid; the anterior side broad, very open, 

 covered by two shield-shaped, accessory plates, and strongly 

 arcuated, with echinated stria?, separated from the smooth part 

 by a faint depressed line, and terminating in a sharp beak ; the 

 posterior side irregularly striated transversely; umbones placed 

 much to one side, and covered by a large, heart-shaped, 

 smooth, accessory process ; by the side of which is a narrow, 



