156 Annals of the South African Museum. 



This also has very faint and delicate radial, linear ornamentation on 

 the anterior part of the flank, but the African shell may originally 

 have borne similar minute sculpture, and on this point nothing 

 further can at present be said. 



Similar forms were described by d'Orbigny, under the name 

 Solen, from the Lower Cretaceous (S. robinaldinus) * and from the 

 Chalk (S. cequalis).\ Gueranger has figured a shell from the Ceno- 

 manian of the Sarthe under the name Solecurtus cequalis,l to which 

 our specimen appears to bear a close resemblance, so far as com- 

 parison is possible. 



GENUS MACTEA Linn. 



MACTRA ? DUBIA sp. nov. 



Plate VIII., figs. 2, 2a, 3, 3a. 



Description. The shell has greater length than height, and the 

 umbones are situated at rather more than one-third of the shell's 

 total length from the anterior extremity. The inflation is moderate, 

 and most strong just above the middle of the valves. The umbones 

 are somewhat weakly developed and are not very prominent. The 

 cardinal margin and the posterior margin form together a curved 

 outline which passes down, posteriorly very steeply, to a marked 

 angular junction with the lower border. From the umbo a well- 

 marked carina passes obliquely across the posterior part of the 

 valve, down to the postero-inferior angle of the valve-margin. The 

 carina marks off a flattened or very slightly concave postero-superior 

 area which occupies less than one-quarter of the total valve-surface. 

 In front of the umbo the valve-margin slopes down to form a some- 

 what sharply curved outline in front, the foremost point of the 

 anterior margin falling well within the lower half of the shell. The 

 long pallial margin shows a broadly convex outline, sometimes 

 slightly flattened towards the posterior end. The greatest height of 

 the valve occurs at the umbonal part. The surface is smooth, with 

 numerous delicate growth-lines, and shows closely spaced radial 

 rows of very minute punctse, only visible under a lens. 

 Dimensions. (1) (2) 



Length 15 . 15mm. 



Height, measured from the umbo 12 12 ,, 

 Greatest depth of a single valve 4 . 3 '5 ,, 



* d'Orbigny (3), p. 320, pi. 350, figs. 1, 2 (1845). 

 t d'Orbigny (3), p. 321, pi. 350, figs. 5-7 (1845). 

 I Gueranger (1), pi. xv., fig. 3. 



