AECINELLA. 



73 



near the inner margins ; hinge rectilinear, forming a flat, exter- 

 nal 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 aU other bivalve shells, by the straight, linear row 

 of small, 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 impres- 

 sion. These shells are found recent, in various marine localities ; 

 fossil, in the tertiary deposits. The Area Nose, formerly re- 

 garded as the type of this genus, has, with several other species, 

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

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

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

 The true Arcse are mostly tropical. ' Mr. Eeeve's Monograph 

 contains 122 species. Area Antiquata, fig. 131. Byssoarca 

 Nose, 132. PI. vii. 

 AECACEA. 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. Aeca. Hinge straight ; valves close. !Fig. 131. 



2. Btsso-aeca. Yalves gaping. Kg. 132. 



3. Circiri/ii^A. Distant teeth oblique ; posterior muscular 



impression prominent. Fig. 133. 



4. Pectttncttlits. Hinge curved. Pig. 134. 



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



including Myopara and Crenella. Pigs. 135 to 137. 

 6L SoiENELXA. Presh water, oval ; a series of teeth on one 



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



Pig. 138. 

 AECHAIAS. Montf. A genus of microscopic Poraminifera. 

 AECHONTE. Montf Htal^a, Auct. ? 

 AECINELLA. Schum. 1817. Acanthiza, Pischer. 1807. 



