PART II. ATRACHIA. -AVICULID^. 



125 



583 



PECTUNCULUS Lam. Orbicular ; the bosses central ; 

 hinge margin curved, with a line of small oblique 

 teeth diverging on each side. (fig. 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 j the 

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



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



BYSSOARCA f Sto. 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. Nose. Sow. Gen. f. 132. 



TRIGONIA Lam. Shell perlaceous, sub-cardiform, - one 

 extremity rounded, the other truncated and com- 

 pressed ; cardinal teeth |, sagittate, compressed, and 

 regularly grooved: intermediate between Bysaoarca 

 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 Cucull<e.a. 

 t The above remark is also applicable to this group. 



