331 



subeqiial, the anterior being rounded, the posterior divided ; 

 impression of the posterior adductor of the byssus elongate and 

 situated near the cardinal line ; that of the anterior adductor of 

 the byssus small ; pallial line simple. 



The genus has been much subdivided according to the character 

 of the external ornament, the shape of the shell, and the nature 

 of the ligament area. 



Type. — Area noce, Linna3us. 



Area equidens, Tate. 



1886. Area equidens, Tate, Trans. Eoy. Soc. South Aust. vol. viii. p. 139, 

 pi. xi. fig. 9. 



In this small species the valves gape ventrally ; -whilst the 

 umbones are high and wide apart, and the few hinge teeth are 

 oblique. There is a characteristic depression from the umbo to 

 the ventral margin ; surface ornament bold, consisting of concentric 

 imbricating ridges and acute radial costae. The author remarks 

 that the " ornament varies in respect to the relative strength 

 of the radial ribs and concentric ridges, whilst in the other extreme 

 it consists of radial ribs variously thickened at the intersections 

 with the concentric lines." The inner margin of the valves is not 

 crenulated; hinge area broad, longitudinally lineated. 



Judging from the obliquity of the hinge teeth and their increase 

 in size laterally, this species forms a connecting link between Area 

 (^sen-iu stricto) and the subgenus Barlatia. 



Dimensions. — Height 3-5 mm. ; length 6 mm. 



Form, and Loc. — Eocene : Aldinga, South Australia. 



L. 9872. Several specimens of different stages of growth. 



Purchased. 



Subgenus BARBATIA, Gray. 

 [Syn. Brit. Mas. 1840, p. 151 (list name) ; Proc. Zool. Soc. 1847, p. 197.] 



Teeth numerous, those in the centre under the umbo small and 

 increasing in size laterally, more or less oblique ; ligament area 

 naiTow. 



2'ype. — Area harlata, Linnajus. 



