Nigerian Eocene Mollusca. 97 



dorso-marginal sulcus, which probably received the ligament as well as the 

 postero-lamellar tooth of the opposing valve. The chondrophore surface is 

 covered with minute and closely -set concentric striations crossed by radial riblets, 

 and bordering three-fourths of its anterior side (which is longest) is a narrowly 

 elongate, rounded, funnel-shaped cavity marked with fine concentric and radial 

 striations, which is regarded as the receptacle for the anterior part of the 

 resilium. This cavity bears an antero-marginal ridge which may probably repre- 

 sent the so-called anterior lamellar tooth of the original description of the species ; 

 then follows the dorso-anterior marginal sulcus which holds the ligament. The 

 basal or underneath surface of the hinge-plate is quite smooth, more or less 

 bulbous, and furrowed. Right valve with a slightly excavated, triangularly-oval 

 chondrophore, vertically situated, and merged in the sub-umbonal region, its 

 surface similarly ornamented to that of the other chondrophore, although smoother 

 and less obvious. The posterior border of this cavity is obliquely and roundly 

 ridged to fit the postero-lateral groove of the left chondrophore, this being 

 followed by an oblique sulcus and a prominently oblique, postero-laminar tooth, 

 to which succeeds a wide and flattened internal dorsal margin. Neither of the 

 valves exhibits any indication of cardinal teeth. 



The pallial impression as seen on internal casts shows a large and nearly 

 circular sinus, an adult specimen measuring 85 mm. in length giving the dimen- 

 sions of this sinus as 25 by 28 mm. in vertical and longitudinal diameters 

 respectively. 



The adductor scar impressions are of very unequal position and shape, the 

 posterior being of moderate size, oblong-pyriform, concentrically striated, and 

 well above the middle of the shell margin, while the anterior is narrowly 

 elongate, extending to nearly the antero-ventral curvature of the valves, as in 

 Raeta. 



The sculpture consists of rounded plications and furrows marked with 

 extremely fine concentric interlineations, together with obscure microscopical 

 radial striations which enter into the texture of the shell ; the surface is also 

 wrinkled, especially posteriorly, caused in all probability by a periostracum which 

 must have originally covered the valves as in Raeta. Sometimes the plications 

 are irregular, and occasionally they bifurcate midway across the valve, terminating 

 singly in the posterior region, when they become more distant and consequently 

 fewer. 



7 



