90 Annals of the South African Museum. 



the adult. The umbones are situated at almost one-third of the 

 shell's total length from the anterior extremity. The umbonal 

 region is relatively very inconspicuous, and of blunt, rounded form. 

 The shell-substance is for the most part very thick, but becomes 

 attenuated at the pallial margin. The upper margin in front of the 

 umbo passes by a curve into the rather short, convex anterior border. 

 The posterior margin merges into the upper margin and slopes 

 obliquely back, to pass by an abrupt curve into the inferior 

 margin. 



The greatest height is at the umbo, and the greatest inflation, 

 which is relatively weak, occurs below the umbonal region, in the 

 superior half of an adult individual. About the middle of the shell 

 near the pallial margin the valves appear slightly constricted, as 

 though laterally pinched in. The surface is marked by numerous 

 noticeable furrows and ridges of accretion. 



Dimensions. (1) (2) 



Greatest length 37 . 43mm. 



Greatest height 21 . 23 ,, 



Greatest depth of a single valve 7 . 8 ,, 



Occurrence. In a hard calcareous band in the rocks of Ather- 

 stone's " Wood Bed " series, on the north bank of the Bezuiden- 

 hout's Eiver below Blue Cliff station (322, 323). 



Remarks. The shells still retain traces of the strong periostracum, 

 and the elongated external ligament is also in part preserved. This 

 is apparently the form cited by Messrs. Eogers and Schwarz as 

 Psammobia,'"'- and such a generic determination might appear to 

 receive some support from the elongated oval figure of the shell, the 

 inconspicuous umbones, the relatively compressed aspect of the 

 valves, and the lengthened external ligament. The valves, how- 

 ever, are very thick, and this character, as well as the wrinkled 

 surface and the blunt and obviously corroded umbones, proclaims 

 another generic position ; moreover, the pallial line is entire. In 

 the shell-structure, also, although this has become obliterated in its 

 intimate characters through replacement by calcite, it can be clearly 

 observed that a line of demarcation separates a thin outer layer, 

 representing the original prismatic layer, from the relatively thick 

 inner portion, formerly consisting of nacreous substance. 



This form differs from Unio porrcctus J. de C. Sow.,f from the 

 Wealden, by the less strongly elongated outline and the much less 

 tapering posterior extremity. Unio antiquus J. de C. Sow. 



* Rogers and Schwarz (1), p. 13. 



t J. de C. Sowerby (1), vol. vi., Tab. 594, fig. 1 (1828). 



