ATRACKIA. - AVICUMD^. 



S83 



Pectunculus Lam. Orbicular : the bosses central ; 

 hinge margin curved, with a line of small oblique 

 teeth diverging on each side. (^fiy. 125. a.) 

 P. pilosus. Sow. Man. f. 134. 



Arc A. Shell of various shapes, either equivalve or in- 

 equivalve, but the valves closing all round ; the bosses 

 very prominent ; hinge margin always straight ; the 

 teeth small.* (fig. 125. b,d.) 



A. antiquata. Sow. Man. f. 131. 



Byssoaroa t Sw. Shell always transversely oblong or 

 oval, generally angulated ; hinge margin straight, 

 often auriculated ; the teeth minute ; valves gaping 

 on the basal margin, (fig. 125. c, e.) 

 B. Zebra. Zool. 111. ii. pi. 113. Noa?. Sow. Gen. f. 132. 



Trigonia Lam. Shell perlaceous, sub-cardiform, one 

 extremity rounded, the other truncated and com- 

 pressed ; cardinal teeth f, sagittate, compressed, and 

 regularly grooved: intermediate between Byssoarca 

 and Nucula, and representing Cardium, Pandora, 

 and all the other cardiform types. 



T. pectinata. Sow. Gen. Man. f. 139. 



* The sub-genera require to be made out : one of these is Cucullaa. 

 t The above leraark is also applicable to this group. 



