54 LAMELLIBRANCHIATA. 
the umbones incurved; hinge formed of a series of transverse 
grooves. Obs. The larger valves of these fossil shells resemble 
the larger valve of Gryphza; but the hinge is quite distinct. 
The species described in Mineral Conchology are found in the blue 
Marl, at Folkstone, and in the chalk. I. Lamarkii. Cariuuvs, 
Brogn. Fig. 167. 
JODA’MIA. Defrance. A genus resembling Birostrites, except 
that in Jodamia one valve overwraps the other, while in Biros- 
trites the circumference of the valves is equal. 
IRIDI'NA. Lam. A genus belonging to the Nayades, and resem- 
bling the Anopont#, Auct. but its peculiar characteristic is 
that the hinge lamina is tuberculated or crenulated in its whole 
length. Sowerby unites all the genera of the family in the 
genus Unio. Fig. 150, I. Elongata. 
IRRE’GULAR Shells. Are those which, being attached to, or 
imbedded in other marine bodies, have no constant form, but 
are modified in shape according to the substances to which they 
are fixed, as the Chamacea, fig. 153 to 155. 
Y’RUS. Oken. Comprehending Panpora, PETRICcOLA, Saxicava, 
&e. 
ISOCAR DIA. Lam. (ios, isos, similar; Kapéia, cardia, heart.) 
Fam. Cardiacea, Lam. Camacea, Bl. Deser. Cordiform, regular 
equivalve, ventricose, with distant, diverging, involute, free um- 
bones; hinge with two, compressed cardinal, and one, distant, 
compressed, lateral tooth in each valve; ligament external, bifid, 
diverging in the direction of the umbones. Cbs. The shells 
composing this genus are remarkable for the beautiful curva- 
ture of the diverging umbones. Fig. 126, I. Moltkiana. 
KEL‘LIA. Turton. Mya Suborbicularis. Montagu. 
LA’BIUM, or inner lip. Is used to express that side of the aperture 
which is nearest the axis, and generally contiguous to the body 
whorl, the lower part of this, when sufficiently distinct from the 
part which overwraps the body whorl, is called the Columella. 
LA’BRUM, or outer lip. Is the edge of the aperture at the great- 
est distance from the axis. 
LACU’NA. Turt. Fam. Turbinacea, Lam. Pescr. Globose, thin, 
covered with a smooth epidermis; spire short, consisting of 
few rapidly increasing whorls; aperture semilunar, rounded at 
the extremities; columella oblique, reflected over part of the 
umbilicus; umbilicus forming a lengthened area behind the 
columella. Fig. 364, L. Pallidula. 
LAGEN’ULA. Montf. A genus of microscopic Foraminifera. 
LA’MELLATED. (Lamella, a thin plate.) When the layers of 
which a shell is composed, instead of beg compacted into a 
solid mass, are separated, overlying each other with the edges 
produced, the structure is said to be lamellated or foliaceous. 
LAMELLIBRANCHIA’TA. Bl. The third order of the class 
