84: Annals of the South African Museum. 



this form can at present only be looked upon as a representative 

 of the genus Hytilus. No doubt, indeed, concerning its generic 

 position could have been reasonably suggested, had not a detailed 

 study of the externally very similar Mytilus lanceolatus J. de C. 

 Sowerby, led Mr. H. Woods to ascribe that shell to the genus Dreis- 

 sensia," a determination which was based upon the presence of an 

 umbonal septum. A strong case was made out by Mr. Woods for 

 the existence of marine forerunners of the modern Dreissensia, an 

 inhabitant of brackish and fresh waters, and he was also supported 

 by the opinion of M. Cossmann, who has found the genus to be 

 associated with marine forms in the Upper Eocene of the Paris 

 basin. In view of close external similarity to Driessensia lancco- 

 lata it would not be surprising if the shell here described were 

 eventually also shown to be furnished with an umbonal plate, 

 but until this can be proved it will be necessary to retain the name 

 Mytilus. 



Compared with Dreissensia lanceolata (= Mytilus lanceolatus J. 

 de C. Sow.)t from the Lower Greensand and Upper Greensand and 

 Blackdown Beds of England, M. uitenliagensis is seen to agree closely 

 in general form, but to be distinguished by several points of detail. 

 In the African shell the antero-ventral area is flat throughout its 

 extent, which is not the case in the European form ; in M. uiten- 

 hagensis the carinal ridge is more sharply acute in the umbonal 

 region than in many examples of D. lanceolata. Further distin- 

 guishing features possessed by M. uitenliagensis are the relatively 

 slightly shorter hinge-line, the less steeply sloping outline of the 

 posterior margin, the relative narrowness of the flank between the 

 carinal ridge and the posterior margin, and the absence of con- 

 centric ornaments at any stage of growth. The shell from the 

 Cretaceous of Shingle Bay, Skidegate Inlet (Queen Charlotte 

 Islands), described by Whiteaves under the name Mytilus lanceo- 

 latus, I may or may not be identical with the European form. 

 To judge from the description and figures, it shows considerable 

 similarity to M. uitenliagensis, though appearing to differ by the 

 imperceptible passage of the hinge-border into the posterior border, 

 the curved outline of the carinal margin, and the concavity of the 

 antero-ventral area. 



Several forms described from the Senonian Greensand of Aachen 



* Woods (3), vol. i., p. 110 (1900). 



f J. de C. Sowerby (1), vol. v., p. 55, Tab. 439, fig. 2 (1823); H. Woods (3), 

 vol. i., p. 110, pi. xviii., figs. 13-15, pi. xix., figs, 1-11 (1900). 

 } Whiteaves (3), p. 236, pi. 31, figs. 7, la. 



