258 THE LAMELLIBRANCHIA 



SUB-ORDER 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. 



FAMILY 1. ARCIDAE, 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 Area, Linnaeus ; foot byssiferous ; heart above the 

 rectum ; hinge straight (Figs. 188, 199) ; British. Pectunculus, 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, 

 Gray. Senilia, Gray. Anadara, Gray. Adacnarca, Pelseneer. FAMILY 2. 

 PARALLELODONTIDAE, Ball. 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. FAMILY 3. LIMOPSIDAE, Dall. Shell sub-orbicular, the hinge curved, 



the ligament simple with the trans- 

 verse axis longer than the longitudinal ; 

 foot elongate, pointed anteriorly and 



i i ^^jgffri^' N posteriorly. Genera Limopsis, Sassi ; 



shell covered with a hairy epidermis ; 

 the anterior adductor frequently much 

 reduced (Fig. 233). Trinacria, Mayer ; 

 from the Tertiary. FAMILY 4. PHILO- 

 BRYIDAE, Bernard. The animal, like 

 that of LimopsiSj without an anterior 

 adductor muscle ; the shell thin, very 

 inequilateral, the anterior part atro- 

 phied, the umbones projecting and 

 FIO. 233. formed by the prodissoconch. Genera 



Umo 1 i,longitilosa,p*}*., interior aspect Philobrya, Carpenter (Figs. 196, 

 of the right valve 1 . a.o, anterior adductor 2 34). FAMILY 5. CYRTODONTIDAE, 

 impression; I, hgamentar fossa; p.a, pos- _-.*/ . ., ... 



terior adductor imprewion ; 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. Cypricarditcs, Conrad ; Silurian. Vanuxemia, Billings ; 

 Silurian. FAMILY 6. TRIGONIIDAE, Fleming. Foot elongated, pointed 

 in front and behind, the ventral border sharp. The byssogenoue 

 apparatus atrophied and devoid of a byssus. The labial palps distinct 

 from the lips. Shell thick. Hinge with striated teeth. Genera 

 Trigonia, Bruguiere ; shell sub-triangular, the umbones directed back- 



