92 DIMY ARIA. MYOPARA. 
lateral ones very distinct, one of which is situate close to 
the superior end of the cardinal tooth, and a little behind 
it ; the other somewhat more distant, with a minute one 
between it and the cardinal tooth; mantle muscular im- 
pressions almost entire ; ligament marginal, thick, rather 
short, and external. 
Trigonia costata. Plate VIII. fig. 29. TZ. politus. 
Plate VII. fig. 27. 
Only one recent species of this genus is known, which inhabits the seas of 
New Holland. Many fossil species are found in the Lias, the upper and 
lower Oolites, and Greensand, and characterize the strata above the Lias, and 
below the Chalk. 
TRIBE ITI.— ARCACEA. 
Shells provided with numerous small primary teeth, dis- 
posed in a straight or interrupted line in each valve. 
Genus VI. — NUCULA. — Lamarck. 
Generic Character. — Shell equivalve, inequilateral, trans- 
verse, oval, trigonal. or oblong; generally covered with an 
epidermis ; hinge linear, narrow, divided into two parts by 
an oblique, produced, nearly central pit, which is destined 
for the reception of the ligament, the one anterior, and the 
other posterior ; lateral teeth on each side numerous, acute, 
elevated, somewhat recurved, those of the opposite valves 
locking into the intervening spaces ; umbones contiguous, 
and not separated by an intervening area ; two simple mus- 
cular impressions; mantle impression destitute of a sinus. . 
Nucula Cobboldie, Plate VIII. fig. 20. 
The shells of this genus are marine. Many fossil species have been dis- 
covered ; they occur in the Crag, London Clay, and Chalk Marl, of England, 
Franee, and Italy; especially in the Calcaire-grossier of Bordeaux, Volognes, 
and Paris ; and also in our Greensand. 
In the species N. lanceolata, fluviatilis, pella, rostrata, tellinoides, and 
oblonga, there is a small sinus in the muscular impression of the mantle. 
Genus VII. — MYOPARA. — Lea. 
Generic Character. — Shell inequivalve, inequilateral, 
subtransverse; beaks elongated and incurved ; teeth nume- 
rous, placed in a divergent, interrupted series on each side 
of the pits, formed for the reception of the ligament; mar- 
gins of valves smooth. 
Myopara costatus. Plate XI. fig. 4. 
Known only in a fossil condition, in the United States of America. 
a ee 
