258 THE LAMELLIBRANCHIA 
SuB-OrRDER 2. ARCACEA. 
Symmetrical animals, with the mantle open throughout its extent, 
and with generally well-developed anterior and posterior adductor 
muscles. The heart lies in the pericardium and gives off two aortae. 
The gills are free and without interlamellar junctions. The renal and 
genital orifices are separate. 
Famity 1. ArcipAr, Gray. The borders of the mantle bear com- 
pound pallial eyes. The labial palps are direct continuations of the lips 
(Fig. 199). The hinge is ‘“pliodont,” that is to say, it has numerous 
teeth on either side of the umbones, and the teeth are perpendicular to 
the edge. Genera—Arca, Linnaeus ; foot byssiferous ; heart above the 
rectum ; hinge straight (Figs. 188,199); British. Pectwneulus, Lamarck ; 
foot without byssus, but with a plantar surface ; the heart traversed by 
the rectum ; the hinge curved ; British (Fig. 193, A). *Scaphula, Benson ; 
from fresh water; India. Argina, Gray. Bathyarca, Kobelt. Barbatia, 
tray. Senilia, Gray. Anadara, Gray. Adacnarca, Pelseneer. Famity 2. 
PARALLELODONTIDAE, Dall. The shell of Arca, but with the posterior 
hinge teeth elongated and parallel to the cardinal border. Genera 
Cucullaea, Lamarck ; recent and fossil from the Jurassic. All the other 
genera are fossil, e.g. Parallelodon, Meek and Worthen ; from the Devonian 
to the Tertiary. Carbonaria, Meek and Worthen ; from the Carboniferous, 
etc. Famity 3. Lrmopsipag, Dall. Shell sub-orbicular, the hinge curved, 
the ligament simple with the trans- 
verse axis longer than the longitudinal ; 
foot elongate, pointed anteriorly and 
posteriorly. Genera—Limopsis, Sassi ; 
shell covered with a hairy epidermis ; 
the anterior adductor frequently much 
reduced (Fig. 233). Trinacria, Mayer ; 
from the Tertiary. Faminy 4. PHILo- 
BRYIDAE, Bernard. The animal, like 
that of Limopsis, without an anterior 
adductor muscle ; the shell thin, very 
inequilateral, the anterior part atro- 
phied, the umbones projecting and 
Fic. 233. formed by the prodissoconch. Genera 
Limopsis longipilosa, Pels., interior aspect —Philobrya, Carpenter (Figs. 196, 
of the right valve. a.a, anterior adductor 234). Faminty 5. COyYRTODONTIDAE, 
impression ; J, ligamentar fossa; p.a, pos- a 3 2 z 
terior adductor impression ; t, hinge-tooth. Wohrmann. An extinct family with 
an equivalve short, convex and 
inequilateral shell, the anterior side of which is short; the hinge teeth 
oblique or horizontal. Genera — Cyrtodonta, Billings; Silurian and 
Devonian. Cypricardites, Conrad ; Silurian. Vanuxemia, Billings ; 
Silurian. Faminy 6. Tricontmpasr, Fleming. Foot elongated, pointed 
in front and behind, the ventral border sharp. The byssogenous 
apparatus atrophied and devoid of a byssus. The labial palps distinct 
from the lips. Shell thick. Hinge with striated teeth. Genera— 
Trigonia, Bruguiére ; shell sub-triangular, the umbones directed back- 
