CONCHIFERA. 455 
Type, C. Hibernicum, Pl. XIX., Fig. 5. OC. aliforme, 
Fig. 254. 
Shell, equivalve trigonal, conical and gaping infront, truncated 
behind, with a long siphonal tube near the umbones ; anterior 
Fig. 254. Conocardium aliforme, Sby. Carb., Ireland. (Mus. Tennant.) 
slope radiately, posterior obliquely striated; margins strongly 
erenulated within; hinge with anterior and posterior laminar 
teeth ; ligament external. 
The truncated end has usually been considered anterior, a con- 
clusion which seems incompatible with the vertical position and 
burrowing habits of most free and equivalve shells ; if compared 
with Adacna (Fig. 253) the large gape (a) will be for the foot, 
and the long tube (s) siphonal. C. Hibernicum has an expanded 
keel, like Hemicardium inversum. The shell-structure is pris- 
matic-cellular, as first pointed out by Sowerby; but the cells 
are cubical, and much larger than in any of the Aviculade. In 
Cardium the outer layer is only corrugated or obscurely pris- 
matic-cellular. 
Fossil, 30 species. U. Silurian — Carb. North America, 
Europe. 
FAMILY XI.—LUCINIDA. 
Shell orbicular, free, closed; hinge-teeth 1 or 2, laterals 1—1 
or obsolete; interior dull, obliquely furrowed; pallial line 
simple; muscular impressions 2, elongated, rugose; ligament 
inconspicuous or sub-internal. 
Animal with mantle-lobes open below, and haying one or 
two siphonal orifices behind; foot elongated, cylindrical, or 
strap-shaped (ligulate), protruded at the base of the shell; gills 
one (or two) on each side, large and thick, oval; mouth and 
palpi usually minute. 
The Lucinide are distributed chiefly in the tropical and 
temperate seas, upon sandy and muddy bottoms, from the sea- 
shore to the greatest habitable depths. Theshell consists of two 
distinct layers, 
Fig. 255 represents the animal of a species of Diplodonta, 
