128 MOLLUSCA FROM THE CRAG. 



Shell very inequilateral, subrhomboidal or slightly wedge-shaped, compressed 

 or flattened, covered with concentric striae; posterior side very short, angulated, 

 anterior produced, slightly contracted, and rounded ; ventral and dorsal margins nearly 

 straight ; two diverging teeth in each valve, much the larger in the right ; a small 

 triangular ligamental cavity placed immediately beneath the umbo. 



Length, fths. Height, \ of an inch. 



Locality. Cor. Crag, Sutton. 



This species is by no means scarce at Sutton, although valves of the above 

 dimensions are not very often met with. 



There are differences in this shell that seem to justify a removal from the pre- 

 ceding, although it must be confessed it is very nearly related, and in the immature 

 state it is exceedingly difficult, if not impossible, to separate them, but what are con- 

 sidered to be the distinctive characters may be pointed out leaving to future obser- 

 vation to confirm or refute their identity. 



In the recent species {bidentatd), the shell is not only less in size but more 

 tumid, and nearly smooth, our fossil is compressed, and the body of the 

 shell much flattened, the posterior side is particularly short and truncate, sloping 

 direct from the umbo. On the anterior side, which constitutes nine tenths of the shell, 

 the dorsal and ventral margins are nearly straight but not quite parallel, the termi- 

 nation rounded and narrower, giving a somewhat cuneiform shape to the shell; the 

 exterior is covered with regular and large concentric striae or lines of growth, and the 

 ventral portion of the shell is slightly compressed, with an imperfect angular ridge on the 

 shorter side : the right valve has two large diverging teeth, the posterior one being 

 the smaller ; between these and immediately beneath the umbo is a small triangular 

 cavity for the ligament. In the left valve, the margin is elevated into angular 

 denticles which are inserted, when the valves are closed, between the margin and the 

 teeth of the opposite valve. Muscular impressions not very distinct. 



3. Montacuta substriata, Montague. Tab. XII, fig. 12, a, b. 

 Ligula substriata. Mont. Test. Brit. Sup., p. 25, 1808. 

 Mya substriata. Dittw. Desc. Cat. Rec. Shells, p. 47, 1817. 



— — Turt. Conch. Diet., p. 103, 1819. ■ 



_ _ W. Wood. Ind. Test., p. 11, No. 22, 1825. 



Montacuta substriata. Turt. Brit. Biv., p. 59, t. 11, figs. 9, 10, 1822. 



— — Flem. Brit. An., p. 465, 1828. 



— — S. Wood, Catalogue, 1840. 



— — Macgill. Moll. Aberd., p. 303, 1843. 



— — Lovtn. Ind. Moll. Scand., p. 43, 1846. 



— — Alder. Cat. Moll. North, and Durh., p. 96, 1848. 



— — Fori, and Hani. Hist. Brit. Moll., vol. ii, p. 77, pi. 18, figs. 8, 



8a, and pi. O, fig. 2, 1849. 

 Tellimya — Brown. Illust. Brit. Conch., pi. 16, fig. 23, 1827. 



Erycina — Recluz. Rev. Zool., p. 330, 1844. 



Sph^ema costulata. Macgill. Moll. Aberd., p. 301, 1843, fide Forb. and Hani. 



