15 
and sunk under the beak; one elongated lateral tooth, situated under the corselet. 
Ligament external, forked on one side. 
Plate VIII. Fig. 7- («) I- cor. (Chama cor.— Linn.) 
( b ) Hinge of ditto. 
ARC ACE A. — 4 Genera*, 
Cardinal teeth small, numerous, entering, and disposed in each valve, in a straight, arched, or 
broken line , 
Cucull^ea. — Shell equivalve, inequilateral, trapeziform, ventricose; the beaks distant, se- 
parated by the facet of the ligament; the anterior muscular impression forming a 
projecting angular or auricular margin: hinge linear, straight, with small trans- 
verse teeth, and having, at the extremities, from two to five ribs parallel to it. — 
Ligament wholly external. 
Plate VIII. Fig. 8. (a) C. auriculifera. (Area cucullus. — Gmel.) 
( b ) Inside of ditto. 
Arca. — Shell transverse, subequivalve, inequilateral ; the beaks distant, separated by the 
facet of the ligament: hinge in a straight line, without ribs at the extremities, and 
furnished with numerous serial and entering teeth. Ligament wholly external. 
(1) Shells with the superior margin not crenulated within .. 
Plate VIII. Fig. 9. A. Nose. (Idem. — Linn ) 
(2) Shells with the superior margin crenulated within. 
Plate VIII. Fig 10. A. Indica. (Idem. — Gmel.) 
Pectunculus. — Shell orbicular, almost lenticular, equivalve, subequilateral, close : hinge 
arched, furnished with numerous, serial, oblique, entering teeth ; those in the mid- 
dle obsolete, nearly obliterated. Ligament external. 
(1) Shells with distant longitudinal furrows, and often, besides, fine transverse or longitudinal strive . 
Plate VIII. Fig. 11. P. glycymeris. (Arca glycymeris. — Linn.) 
(2) Shells with prominent and radiating longitudinal ribs, with or without transverse strive. 
Plate VIII. Fig. 12. P. pectiniformis. (Arca pectunculus. — Linn.) 
Nucula.- — Shell transverse, ovate-triangular or oblong, equivalve, inequilateral ; no facet 
between the beaks: hinge linear, broken, many -toothed, interrupted in the mid- 
dle by an obliquely extending spoon-shaped pit ; the teeth numerous, often pro- 
duced as in the pectines; the beaks contiguous, curved backwards. Ligament 
marginal, and partly internal, inserted in the pit or spoon of the hinge. 
Plate VIII. Fig. 13. (a) N. margaritacea. (Arca nucleus.— Linn.) 
( b ) Inside of ditto. 
TRIG ONI A X A. _ 2 Genera. 
Cardinal teeth lamellar, transversely striated. 
Trigonia. — Shell equivalve, triangular, sometimes suborbicular ; cardinal teeth oblong, 
laterally compressed, diverging, transversely furrowed; two in the right valve, 
