Mactracea.] 



MOLLUSCA. 



105 



1. Corbula NUCLEUS, pi. XLII, f. 7, 8, 9- 



Corbula nucleus, First Ed., pi. 14, f. 7, 8, 9; Lamarck, V, p. 

 496 ; Turton, Biv., p. 39, pi. 3, f. 8, 9, 10 ; Corbula striata, 

 Fleming, p. 425 ; Mya inequivalvis, Montagu, p. 38, pi. 26, f. 7 > 

 Maton and Rackett, p. 40, pi. 1, f. 6. 



Shell subtriangular, strong, thick, opaque, white ; with an 

 umber-brown, rather thick epidermis; under valve very convex, 

 with a large, tumid, inflected umbo, considerably elevated above 

 the hinge ; upper valve flat, or somewhat concave, with a small 

 but acute umbo, and little more than half the size of the 

 other, provided with a few short, distant, irregularly divergent 

 striae, which emanate from near the central disk of the shell, 

 and terminate on the margin ; larger valve with one strong, 

 erect, somewhat recurved tooth, which is received into a socket 

 in the smaller valve ; inside smooth, dull white ; the margin 

 somewhat flattened, and that of the larger valve projecting con- 

 siderably beyond the other, when closed. 



This species is by no means uncommon on most of the 

 coasts of Britain and Ireland. It attains a large size in the 

 latter country. 



2. Corbula ovata, pi. XLII, f. 32, 33. 

 Corbula ovata, Forbes, p. 53, pi. 2, f. 8, 9. 



" Shell ovata-oblong, almost equilateral, with obsolete trans- 

 verse striae ; white, covered with a brown epidermis, which is 

 worn away at the upper part of the shell, but bears the traces 

 of regular, transverse stria? towards the margin ; the anterior 

 extremity is slightly truncate, the posterior rounded; beaks de- 

 pressed ; translucent. 



" In form this species resembles the Corbula rosea, but is 

 more elongated transversely ; the teeth are very similar. 



" Taken from the root of a Fucus, cast ashore at Ballaugh, 

 Isle of Man." — Forbes. 



3. Corbula rosea, pi. XLII, f. 6. 



Corbula rosea, First Ed., pi. 42, f. 6 ; Leach, MSS. 



Shell subtriangular, subovate, subcompressed, its breadth con- 

 siderably exceeding its length ; under valve but a little larger 

 than the other, its umbo of moderate size ; surface smooth, 

 covered with a very thin, pale yellow epidermis, beneath which 

 the shell is of a beautiful roseate hue, with a few short radia- 

 tions of a darker cast, near the umbones ; inside smooth, of a 

 pale rose-colour ; margins smooth, and glossy. 



This shell differs considerably from the young of C. nucleus, 

 in being devoid of the peculiar curve of that shell; in the beaks 

 of the lower valve being much less, and a less disparity in the 

 size of the upper and under valve ; and in being much more 

 compressed. 



In the cabinet of Dr. Leach, British Museum, and were 

 dredged at Falmouth. 



Family VII — Mactracea. 



Shells equivalve, frequently gaping at the lateral extremities ; 

 ligament situate interiorly, or partly external. 



Section I. — Ligament seen externally, or double. 



Genus 31. — Amphidesma Lamarck. 



Shell equivalve, transverse, slightly inequilateral, somewhat 

 ovate, or orbicular; some species gaping at the sides, with a 

 slight posterior fold ; each valve provided with one, or in some 

 2 D 



instances two small, slender, cardinal teeth ; and two distinct, 

 elongated, lateral teeth, situate near the hinge in one valve, and 

 almost obsolete in the other ; ligament double, separated from 

 the cartilage, its external portion slender, and rather short, 

 the internal cartilage generally longer and larger, adherent in 

 both valves to an elongated, oblique groove or pit, which varies 

 in length in different species, takes its rise immediately within 

 the umbo, and is prolonged within the anterior lateral tooth ; 

 two muscular impressions in each valve ; pallial impression with 

 a very large sinus. 



1. Amphidesma Boysii, pi. XLII, f. 3. 



Ligula Boysii, First Ed., pi. 14, f. 3; Lamarck, V, p. 491 ; 

 Amphidesma Boysii, Turton, Biv., p. 53, pi. 5, f. 4, 5; Mactra 

 Boysii, Montagu, p. 98, pi. 3, f. 7 ; Maton and Rackett, p. 72. 



Shell oval, subcompressed, thin, pellucid, glossy, white, and 

 nearly smooth ; with a few remote, obsolete, concentric striae ; 

 umbones very small, with the beaks slightly inflected, situate 

 nearest to the posterior side ; one cardinal tooth in each valve, 

 with a broad, cartilage cavity ; one valve only provided with 

 broad, elevated, lateral teeth; inside smooth, glossy, and white; 

 two muscular impressions ; the pallial impression with a large, 

 nearly obsolete sinus, running from the smaller side, almost 

 two-thirds across the valves, and nearly parallel with the mar- 

 gin. Length somewhat more than half an inch; breadth a little 

 more than three-quarters. 



Plentiful on most of the British and Irish coasts. 



2. Amphidesma tenue, pi. XLII, f. 2. 



Ligula tenuis, First Ed., pi. 14, f. 2; Mactra tenuis, Mon- 

 tagu, p. 572; Ligula tenuis, Montagu, Sup., pi. 17, f. 7; Amphi- 

 desma tenue, Turton, p. 53; Forbes, p. 47; Fleming, p. 433. 



Shell subtriangular, compressed, equilateral, semipellucid, 

 thin, white ; with shallow, concentric wrinkles ; umbones small, 

 considerably produced, placed nearly central, and not turning 

 to either side ; inside moderately smooth, and glossy ; pallial 

 impression interrupted by a broad, conical, nearly perpendicular, 

 pointed sinus, reaching nearly two-thirds up the valves ; hinge 

 provided with a cleft tooth, a narrow groove, and remote, lami- 

 nated, lateral teeth, one on each side in one valve ; the other 

 with a plain, single tooth, which is received into a cavity in the 

 opposite valve; destitute of lateral teeth. Length a quarter of 

 an inch ; breadth somewhat more. 



Found on the coasts of Hampshire, Dorset, and Devon. 



3. Amphidesma prismatica, pi. XLII, f. 5. 



Ligula prismatica, First Ed., pi. 14, f. 5; Montagu, Sup., p. 

 23, pi. 26, f. 3 ; Amphidesma prismaticum, Fleming, p. 432. 



Shell transversely oblong-ovate, flat, thin, fragile, glossy, pel- 

 lucid, white, exhibiting prismatic reflections ; with very minute, 

 concentric stria; ; umbones very small, inflected, and placed 

 towards the narrow side, which is considerably acuminated, with 

 a slight oblique subtruncation, below which it is pointed; oppo- 

 site side finely rounded ; hinge furnished with a horizontal 

 tooth, with a cavity in the centre for the reception of the con- 

 necting cartilage, and the addition of a small, lateral, laminar ; 

 remote tooth on each side of the larger tooth, in one valve, but 

 devoid of these in the other valve. Length three-eighths of an 

 inch ; breadth upwards of six-eighths. 



Found pretty plentifully in the Frith of Forth, the south 

 coast of Devon, and by no means rare at Portmarnock, Ireland, 

 and other parts of the coast. 



