72 ARCINELLA. 



inequilateral, thick, ventricose, longitudinally ribbed, dentated 

 near the inner margins ; hinge rectilinear, forming a flat, external 

 area, upon which the ligament is spread in cross rows, and 

 having a series of small, regular teeth, extending on both sides of 

 the umbones in each valve ; muscular impressions distant. — 

 Obs. The shells composing this genus are easily distinguished 

 from those of all other bivalve shells, by the straight, linear row 

 of smal], notched teeth, and by the area between the umbones. 

 The genus Cucullcea makes the nearest approach to it in this 

 respect, but it may easily be known from it by the outermost 

 teeth on each side of the row being oblique, and lengthened out; 

 and also by the prominent edge of the muscular impression. 

 These shells are found recent, in various marine localities ; fossil, 

 in the tertiary deposits. The Area Nose, formerly regarded as 

 the type of this genus, has, with several other species, been 

 separated from it under the name of Bysso-arca, by Swainson, 

 on account of an hiatus in the ventral margin, to admit the 

 passage of a byssus ; this is not found in the true Arcse. The 

 true Arcae are mostly tropical. Area Antiquata, fig. 131 . Bysso- 

 arca Nose, 132. 

 ARCACEA. Lam. A family of the' order Conchifera Dimyaria, 

 characterized by a series of teeth placed on the hinge in a line. 

 The genera may be distinguished as follows, 



1. Arca. Hinge straight ; valves close. Fig. 131. 



2. Bysso-arca. Valves gaping. Fig. 132. 



3. Cucull^a. Distant teeth oblique ; posterior muscular 



impression prominent. Fig. 133. 



4. Pectunculus. Hinge curved. Fig. 134. 



5. Nucula. The same, with a pit in the centre of the hinge, 



including Myopara and Crenella. Figs. 135 to 137. 



6. Solenella. Fresh water, oval; a series of teeth on one 

 side of the hinge, only two or three on the other. 

 Fig. 138. 



ARCHAIAS. Montf. A genus of microscopic Foraminifera. 

 ARCHONTE. Montf. Hyal^a, Auct. 

 ARCINELLA. Schum. Chama Arcinella, Auct. 



