ARCID^. 253 



Shell equivalve or iiearl}^ so, thick, snbquadrate, ventricose, 

 strono-ly ribbed or cancellated; margins smooth or dentated, 

 close or sinuated ventrally ; hinge straight, teeth very numerous, 

 transverse ; umbones anterior, separated by a flat, lozenge-shaped 

 ligamental area, with numerous cartilage-grooves; pallial line 

 simple ; posterior adductor impression double ; pedal scars two, 

 the posterior elongated. 



Animal with a long pointed foot, heeled, and deeply grooved ; 

 mantle furnished with ocelli ; palpi ; gills long, narrow, less 

 striated externally, continuous with the lips ; hearts two, each 

 with an auricle. 



The name Byssoarca was chosen unfortnnateh'" by Swainson, 

 for the typical species of the genus, in which the byssal orifice 

 is sometimes very large. The b^'ssus is a liorny cone, composed 

 of numerous thin plates, occasionall}^ becoming solid and calca- 

 reous ; it can be cast off and reformed with great rapidity. The 

 Areas with close valves often have the left valve a little larger 

 than the right, and more ornate. 



The Byssoarks secrete themselves under stones at low-water, 

 in crevices of rocks, and the empty burrows of boring moUusks ; 

 they are often much worn and distorted. 



A large number of genera and subgenera have been formed 

 out of the Linnaean Area, but as in Helix, Murex and other 

 well-known and well-characterized groups, these groups possess 

 but slight value ; the most important of them ma}^ be considered 

 subgenera, perhaps ; others as sections. 



ARCA, Linn, (typical). Shell oblong, subquadrangular, gaping 

 anteriorly or iuferiorly ; hinge linear, straight, formed of a large 

 number of small pectinated teeth ; ligament external, inserted 

 upon a lozenge-shaped area between the beaks ; beaks high, 

 rather wide apart ; muscular impressions very distinct ; pallial 

 impression entire. A. Nose, Linn. The Areas often anchor 

 themselves b}- means of a strong byssus to rocks or stones. 



BARBATiA, Gray, 1840. Shell oblong, oval or subquadrangular ; 

 surface covered by a rude, caducous epidermis ; hinge-line 

 straight or somewhat curved ; teeth numerous, the central ones 

 smallest, the lateral ones becoming gradually larger and more 

 oblique towards the extremities ; ligament external, inseited 

 upon a narrow surface between the approaching beaks. A. velata, 

 Sowb. (cxxvi, 40). 



POLYNEMA, Conrad, 18^5. Shell transversely trapezoid-ovate, 

 narrowing anteriorly ; cardinal margin descending forward, and 

 meeting the anterior and posterior margins at more or less 

 defined angles ; cardinal area extremelj'^ narrow, with about two 

 deep, divaricating cartilage-furrows ; lateral denticles very! 

 oblique, those on the posterior side more elongated and striated ; 



