750 CLASS XII. 



the North Sea and Mediterranean. Mactra Mvacea CHEMN., LAM., 

 Mactra glauca SPENGLER, POLI, 1. 1. figs, i 3, SPENGLER 1. 1. Tab. in. 

 fig. i ; in the Mediterranean. This genus counts very many species from 

 all seas. Some fossil species of it also are known, which occur in the ter- 

 tiary formations. 



Lutraria LAM. (in part), DESH. Shell transverse, elongate pos- 

 teriorly, flattened, lateral teeth none. 



Sp. Mactra lutraria L., Lutraria dliptica LAM., CHEMN. Concliyl. Tab. 24, 

 figs. 240, 241, &c. This sub-genus is much less numerous than the pre- 

 ceding. The species cited occur also fossil in tertiary formations. 



Family XIX. Myacea. Mantle sacciform, with lobes almost 

 entirely conjoined at the margin, perforated anteriorly alone by a 

 small aperture for the transmission of foot. Foot small. Tracheae 

 mostly coalesced, retractile. Shell mostly gaping. Ligament 

 internal, inserted in a tooth or spoon-shaped process, somewhat 

 prominent at the hinge. Two muscular impressions remote; im- 

 pression of mantle mostly excised posteriorly by a sinus. 



Phalanx I. Osteodesmacea. Cochleariform tooth in each valve, 

 receiving the ligament. Mostly a loose calcareous particle (ossicu- 

 lum DESH.) contained in ligament. Two tracheae often separate. 



Cardilia DESH. (Is this its place ?) 



Note. Habitus of Isocardia. Spec. Isocardia semi-sulcata LAM. and 

 some others ; two are fossil from the more recent strata. Comp. DESHAYES 

 in LAMARCK Hist. nat. des Ani. s. v. e"d. 2, vi. pp. 448 450, and GUEKIN 

 Magas. de Zool. 1844, PI. 99 101. 



Myochama STUTCHBURY. Shell affixed, irregular, inequivalve. 

 Animal with short separate trachea. 



Sp. Myochama anomioldes STUTCHBURY, DESH. Concliyl. PI. 8 bis, figs. 12, 



Thracia LEACH, DESH. Shell free, inecjuivalve, transverse, 

 truncated posteriorly, hiant at both ends, with umbones tumid. 

 Ligament double ; external small. Animal with two tracheae 

 short, separate. 



Sp. Thracia convexa DESH., BLAINV. Malacol. PI. 76, fig. 6, DESH. Con- 

 chyliol. PI. 9, figs, i 6 ; in the N. Sea, Mediter. &c. There are fossil 

 species of this genus in the Jura- and Chalk- formations, and in the tertiary 

 strata. 



