750 CLASS XII. 
the North Sea and Mediterranean. Mactra helvacea CHEMN., LAM., 
Mactra glauca Spencuer, Pout, 1. 1. figs. 1—3, SPpENGLER 1. 1. Tab. m1. 
fig. 1; 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), Desu. Shell transverse, elongate pos- 
teriorly, flattened, lateral teeth none. 
Sp. Mactra lutraria L., Lutraria elliptica Lam., CHEMN. Conchyl. 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 XTX. 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. Trachez 
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 J. Osteodesmacea. Cochleariform tooth in each valve, 
receiving the ligament. Mostly a loose calcareous particle (osstcu- 
lum Dersu.) contained in ligament. Two trachez often separate. 
Cardilia Drsu. (Is this its place ?) 
Note.—Habitus of Isocardia. Spec. Lsocardia semi-sulcata LAM. and 
some others; two are fossil from the more recent strata. Comp. DESHAYES 
in Lamarck Hist. nat. des Ani. s. v. 6d. 2, VI. pp. 448—450, and GUERIN 
Magas. de Zool. 1844, Pl. g99—101. 
Myochama STUTCHBURY. Shell affixed, irregular, inequivalve. 
Animal with short separate trachee. 
Sp. Myochama anomioides SrutcHBury, DrsH. Conchyl. Pl. 8 bis, figs. 12, 
13, &e. 
Thracia Leacn, Desu. Shell free, inequivalve, transverse, 
truncated posteriorly, hiant at both ends, with umbones tumid. 
Ligament double; external small. . Animal with two trachez 
short, separate. 
Sp. Thracia conveca DEsH., Buatnv. Malacol. Pl. 76, fig. 6, Desa. Con- 
chyliol. Pl. 9, figs. 1—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, 
