144 MOLLUSCA FROM THE CRAG. 
teeth. Ligament external, no lunule. Impressions by the adductors ovate; mantle 
mark without a sinus. 
Animal of the form of the shell, with its mantle closed all round, except in front, 
through which a lanceolate-shaped foot is protruded: margin of the mantle with 
plain edges. Siphons are said to be wholly wanting. 
Our recent British species has been examined by Mr. Clark, who has pointed out 
an apparent anomaly by which it differs from the generality of Dimyaria, in being 
wholly destitute of anything resembling siphonal tubes, and without an orifice, except 
the pedal one, he says, for the admission of water to sustain the functions of life. 
Its position among the Zwcinide is at present considered doubtful by the 
Malacologists, in consequence of this anomalous character in regard to the mantle; 
the shell, however, so strongly resembles many of the species of this group, that 
no other position seems so appropriate. 
1. DipLoponra ROoTUNDATA, Montague. Tab. XII, fig. 3 a, 6. 
TELLINA RoTUNDATA. Mont. Test. Brit., p. 70, t. 2, fig. 3, 1803. 
— — Mat. and Rack. Linn. Trans., vol. vill, p. 56, 1807. 
—_ — W. Wood. Ind. Test., p. 22, pl. 4, fig. 77, 1825. 
Mysta rorunpata. Brown. Illust. Brit. Conch., pl. 16, fig. 11, 1827. 
— _— Id. Conch. Text. Book, p. 132, pl. 17, fig. 6, 1837. 
— Monvracur. Leach, MS., fide Brown. 7 
Lucina rotuNDATA. Turt. Brit. Biv., p. 114, pl. 7, fig. 3, 1822. 
— _: Thorpe. Brit. Mar. Conch., p. 73, 1844. 
—— — Forbes. Report on Hgean Invert., p. 180, 1843. 
— — Brown. Brit. Conch., 2d ed., pl. 40, fig. 11. 
— — Reeve. Conch. Icon. Lucrna, pl. vii, fig. 36. 
PsAMMOBIA ROTUNDATA. Flem. Brit. An., p. 438, 1828. 
DreLtoponta rotuNDATA. Phil. En. Moll. Sic., vol. ti, p. 24, 1844. 
—_— as Ford. and Hanl. Hist. Brit. Moll., vol. ii, p. 66, pl. 35, fig. 6, 
and pl. M, fig. 7, 1849. 
Spec. Char. Testd suborbiculari vel trapezoidea, inequilateral, postice latiore et 
longiore, subquadratd, antice rotundatd ; apicibus prominulis ; margine dorsal feré rec- 
tilineo: cardine bidentato. 
Shell suborbicular, or of a roundedly trapezoidal form, equilateral, posterior side 
the longer, broader, and somewhat square, anterior rounded, with slightly prominent 
umbones: dorsal margin nearly straight: hinge with two teeth. 
Length, \th of an inch. 
Locality. Coralline Crag, Sutton, Ramsholt, Sudbourn, Gedgrave. 
Red Crag, Sutton. Recent, Mediterranean, and British Seas. 
This is a shell exceedingly abundant in the Coralline Crag, where specimens 
may be obtained from nearly 14 inch in diameter to those which are less than ath 
of an inch. In the Red Crag it is also found, but less abundantly. The hinge in 
