Myauia.] 



MOLLUSCA. 



109 



This species might easily be mistaken for the young of M. 

 triangularis, but will be distinguished from that shell in being 

 less angulated, in wanting crenulations on the margin, and its 

 breadth proportionably greater ; it is, besides, a considerably 

 smaller shell. 



Found on the coast of Cornwall, the Western coasts, and in 

 Ireland. Forbes says it inhabits the byssi of Modioli from deep 

 water off Douglas, Isle of Man. 



Genus 36 — Lutraria Lamarck. 



Shell equivalve, inequilateral, thin, transversely ovate or 

 oblong ; open at both sides ; the posterior side generally the 

 longer, and always gaping more than the other ; one valve with 

 two thin, laminar teeth, one of which is sometimes compound ; 

 the opposite valve with three teeth, the central one compound 

 in some instances, and the posterior one slender and com- 

 pressed ; ligament internal, situate in a deltoidal, oblique, inter- 

 nally projecting, spoon-shaped pit, with a prominent margin 

 placed next to the teeth in each valve ; two distant, lateral, 

 muscular impressions ; pallial impression with a very large 

 sinus, lying transversely in most species. 



1. Lutraria elliptica, pi. XLIII, f. 2, 3. 



Lutraria elliptica, First Ed., pi. 12, f. 2, 3; Lamarck, V, p. 

 468 ; Turton, p. 65 ; Forbes, p. 54 ; Lutraria vulgaris, Fle- 

 ming, p. 464 ; Pennant, IV, p. 92, pi. 52, f. 44 ; Montagu, p. 

 99 ; Donovan, II, pi. 58. 



Shell transversely oblong-ovate, thin, compressed ; longer 

 side with a shallow groove, emanating from the base of the 

 umbones, and terminating on the margin, over which there are 

 some irregular striso-like lines, which when the epidermis is 

 removed, present the appearance of being scratched by a file ; 

 umbones small, placed near to one side; surface with fine, nearly 

 obsolete, irregular, concentric striae, or grooves, and irregularly 

 wrinkled transversely, these are shallow on the disks, but 

 stronger and more decided towards the sides, covered with a 

 pretty strong epidermis, of a drab-colour, rarely approaching to 

 fawn ; sometimes with a few indications of lines of growth, and 

 in some these assume the appearance of nearly obsolete ridges, 

 particularly towards the shorter side; inside smooth, bluish- 

 white ; and gaping at both sides. 



Found on many of the Biitish coasts with a sandy bottom, 

 especially at the influx of rivers. 



Fig. 3 is a very thick shell, in proportion to its size, which I 

 figured from the collection of Sir John Trevelyan, Bart., at 

 Wallington, Northumberland. 



2. Lutraria Solenoidea, pi. XLIII, f. 1. 



Lutraria Solenoidea, First Ed., pi. 12, f. 1; Lamarck, V, p. 

 468 ; Lutraria oblonga, Turton, Biv., p. 64, pi. 5, f. 6 ; Lutra- 

 ria Mans, Fleming, p. 465 ; Psammophila Solenoidea, Leach, 

 MSS., p. 6 ; Mactra hians, Montagu, p. 101 ; Donovan, III, 

 pi. 140. 



Shell transversely oblong-ovate, greatly inequilateral ; um- 

 bones small, depressed ; surface with rugose, concentric wrin- 

 kles, which prevail most at the posterior side, covered with 

 an opaque, dark brown, thick, dusky epidermis, sometimes 

 of a reddish tinge ; hinge provided with one flattened tooth, 

 which is grooved on the fore part, and locks between two 

 2 E 



in the opposite valve, the smallest of which is cleft ; the car- 

 tilage pit is large and triangular ; inside smooth, white, with 

 an elevated, transverse ridge, situate on the upper side of the 

 posterior muscular impression ; gaping considerably at the pos- 

 terior side. 



An extremely local species, but is said to be not uncommon 

 at the influx of the river between Truro and Falmouth, in 

 Cornwall ; and has been found on the Dorset coast. 



3. Lutraria compressa, pi. XLIII, f. 4. 



Lutraria compressa, First Ed., pi. 12, f. 4 ; Lamarck, V, p. 

 469; Listeria compressa, Turton, Biv., p. 51, pi. 5, f. ], 2; 

 Abra compressa, Leach, MSS., p. 6; Amphidesma compressum, 

 Fleming, p. 432 ; Mactra Listeri, Gmelin, p. 3261 ; Mactra 

 compressa, Montagu, p. 96 ; Tellina plana, Donovan, II, pi. 

 64, f. 1. 



Shell transversely subovate and subtriangular, thin, brittle, 

 much compressed ; umbones not quite central, and rather ob- 

 tuse ; anterior side well rounded ; posterior side a little longer 

 than the other, and slightly acuminated ; external surface of a 

 dull yellowish-white, covered with pretty strong, transverse 

 striae-like wrinkles ; cartilage cavity large, triangular ; primary- 

 teeth small, a single one which locks into a cleft tooth in the 

 opposite valve; inside smooth, white, and rather dull; pallial 

 sinus very large, occupying nearly a half of the interior surface; 

 margin plain. 



A common species on most of the British coasts, and prevails 

 at the mouths of rivers. 



Section IV. — Crassipedes. 



The mouth united in front, or partly so ; foot thick, and 

 situate posteriorly ; shells gaping when closed. 



Family VIII — Myaria. 



Shells with a broad, spoon-like tooth in one or both valves ; 

 gaping at both extremities ; in most instances with an external 

 ligament and internal cartilage. 



Genus 37-— Thracia Leach. 



Shell very thin, transverse, inequivalve, inequilateral, one 

 valve usually more convex than the other ; umbones generally 

 obtuse, and subcentral ; hinge with a broad, transverse, fre- 

 quently thickened tooth in both valves, and an obliquely trian- 

 gular pit in the centre, in which the cartilage is situate; surface 

 covered with a very thin epidermis; two well-marked, but dis- 

 similar muscular impressions in both valves ; pallial impression 

 interrupted by an arcuated sinus, at the posterior side, which is 

 truncated. 



1. Thracia declivis, pi. XLIV, f. 5. 



Anatina declivis, First Ed., pi. 11, f. 5; Anatina pubescens, 

 Turton, Biv., p. 45, pi. 4, f. 1, 2; Thracia declivis, Leach, 

 MSS., p. 5; Mya pubescens, Montagu, p. 40 ; Donovan, II, pi. 

 82; Amphidesma conve.vum, Fleming, p. 431. 



Shell thin, brittle, transversely oblong-ovate; umbones large, 

 very obtuse, and not quite central, that of the larger valve with 

 a hiatus for the reception of the beak of the other valve, and 

 to permit the opening of the shell ; posterior side rounded ; 



