318 Mr. E. A. Smitli on African Molhisca. 



strife, wliich are more or less distinct, and frequently it 

 exhibits more or less of wrinkling or corrugation at the upper 

 part of the valves towards the umbones, which are always to 

 some extent eroded. It is well known that in this genus the 

 amount of wrinkling of the surface is very variable, and 

 therefore cannot be regarded as a reliable specific character. 



In the type figured by Krauss the shell is described as 

 " concentrice ruguloso-striata," and no special reference is 

 made by him to corrugation near the beaks. The apices of 

 his specimens being considerably eroded, it is probable that 

 this feature was for the most part obliterated. The form 

 delineated by Lea under the name of U. Verreauxianus a.\>])Qi\x?, 

 to be precisely that of the type, and although " numerous 

 small undulations at the tip " of the beaks are mentioned by 

 him, no trace of them is discoverable in his figures, and 

 therefore we may assume that they were very insignificant. 



His U. afncanus^ from the same locality as Verreauxiamis, 

 differs from it merely in having the surface smoother, the 

 transverse striae being finer. In form and the character of 

 the hinge they are quite alike. 



The variety named U. natahnsis by Lea, which is the same 

 as U. vaahnsis of Chaper, is peculiar on account of having 

 the upper part of the valves much more strongly wrinkled 

 than the type or the variety africanus, but it agrees with 

 them in general form and the character of the hinge. 



The two obsolete lines radiating backwards from the 

 umbones, referred to by Krauss, exist in all the varieties and 

 specimens examined. The colour of the nacre is as variable 

 as the form and sculpture. li is " pallide carnea " in the 

 type, " salmonis colore tincta " in VerreauxianuSj afi'icann^ 

 and natalensis^ and " teintee en jaune clair " in vaalensis, 

 especially towards the umbones. Finally, there are speci- 

 mens in the National Collection which are olive-brown beneath 

 the beaks, almost white towards tiie front part of the ventral 

 margin, and beautifully iridescent at the posterior end. 



The difference in form is very considerable, even in shells 

 belonging to the same variety ; for example, two specimens of 

 the strongly wrinkled form {natahnsis) have the following 

 measurements : — 



From the above figures it is noticeable that specimen a 

 is much broader in proportion than h. The outline of the 

 two shells is quite different, but the sculpture is the same. 



