ORASSATELLA. DIMY ARIA. 111 
each other. Although the Lutrarie have a tendency to this structure, they 
differ from that genus in their lateral teeth, and by the valves being almost 
entirely close, while in the Lutrariz they gape widely at one end. 
The Amphidesme are Oceanic shells, inhabiting the seas of all countries, 
Fossil species, (if they really are true Amphidesmz) are exceedingly rare, and 
occur in the Cornbrash, Kellowaysrock, and upper lias shale ; the A. decaratum 
belongs to the Cornbrash. 
SUBDIVISION II. — Shell not gaping at the sides ; ligament external. 
Genus XLVI. — ERYCINA. — Lamarck. 
Generic Character. — Shell smooth, transverse, ovate, or 
triangular, equivalve, generally inequilateral ; and one valve 
with two unequal, thick, divaricate, cardinal teeth, with an 
intermediate pit for the reception of the ligament, and two 
oblong, compressed, short lateral teeth, situate near the 
primary teeth; the opposite valve is destitute of cardinal 
teeth, or they are quite obsolete ; in some instances one of 
them is united to the anterior jateral tooth, which is always 
very short, and thus increases its thickness; the lateral 
tooth in this valve is compressed and oblong; two lateral 
pallial muscular impressions, with a small sinus, ligament 
internal, affixed in each valve into a narrow, concave space 
between the teeth, 
Erycina fragilis. Plate X. fig. 13. 
The Erycine are distinguished from the Crassatella, Lutraria and Mactra 
by the manner in which the teeth are situate on each side of the ligamentife- 
rous pit, while in the two latter genera they are both situate on the anterior 
side; it is known from the former genus by the sinus inthe mantle muscular im- 
pression and its distant and compressed lateral teeth. 
The shells of this genus are marine, and have been met with in a fossil state 
in the supercretaceous rocks of Bordeaux and Dax. The species are very rare 
in a fossil condition. 
Genus XLVI. — CRASSATELLA. — Lamarck. 
Generic Character. — Shell thick, equivalve, transverse, 
inequilateral ; external surface generally covered withabrown 
horny epidermis, and more or less transversely grooved ; 
one valve provided with two strong cuneiform, rugose cardi- 
nal teeth, which are sometimes perpendicularly grooved ; 
and one primary tooth in the opposite valve ; lateral teeth 
awanting or nearly obsolete, two strong oblong depressions, 
the one on the anterior side of the umbo somewhat elongated, 
and not so well marked as that in the posterior side; two re- 
mote, lateral, rather oblong muscular impressions ; ligament 
internal, attached to a concave pit situate on the anterior 
