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 Lucinidce 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. DIPLODONTA ROTUNDATA, Montague. Tab. XII, fig. 3 a,b. 



TELLINA ROTUNDATA. Mont. Test. Brit., p. 70, t. 2, fig. 3, 1803. 



Mat. and Rack. Linn. Trans., vol. viii, p. 56, 1807. 

 W. Wood. Ind. Test,, p. 22,' pi. 4, fig. 77, 1825. 

 MYSIA ROTUNDATA. Brown. Illust. Brit. Conch., pi. 16, fig. 11, 1827. 



Id. Conch. Text. Book, p. 132, pi. 17, fig. 6, 1837. 

 MONTAGUI. Leach, MS., fide Brown. 



LUCINA ROTUNDATA. Turt. Brit. Biv., p. 114, pi. 7, fig. 3, 1822. 

 Thorpe. Brit. Mar. Conch., p. 73, 1844. 

 Forbes. Report on JSgean Invert., p. 180, 1843. 

 Brown. Brit. Conch., 2d ed., pi. 40, fig. 11. 

 Reeve. Conch. Icon. LUCINA, pi. vii, fig. 36. 



PSAMMOBIA ROTUNDATA. Flem. Brit. An., p. 438, 1828. 

 DIPLODONTA ROTUNDATA. Phil. En. Moll. Sic., vol. ii, p. 24, 1844. 



Forb. and Hani. Hist. Brit. Moll., vol. ii, p. 66, pi. 35, fig. 6, 

 and pi. M, fig. 7, 1849. 



Spec. Char. Testa suborbiculari vel trapezoidea, incequilaterali, postice latiore et 

 longiore, subquadratd, antice rotundatd ; apicibus prominulis ; margine dorsali fere rec- 

 tilineo: cardine bidentato. 



Shell suborbicular, or of a roundedly trapezoidal form, inequilateral, posterior side 

 the longer, broader, and somewhat square, anterior rounded, with slightly prominent 

 umbones : dorsal margin nearly straight : hinge with two teeth. 



Length, l|-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 1 \ inch in diameter to those which are less than |-th 

 of an inch. In the Red Crag it is also found, but less abundantly. The hinge in 



