98 Annals of the South African Museum. 



width. In the adult T. kraussi, within a space of 33 mm. measured 

 backwards along the flank from the lower end of the frontal margin, 

 three ribs only are included. At a height, measured from the umbo, 

 which represents the full adult height of T. ventricosa, the anterior 

 ribs on the flank of T. kraussi are already much more robust and 

 more widely spaced than in T. ventricosa. The anterior inflation is 

 also much stronger. These very well-marked differences in characters 

 developed at a comparable stage in the two forms lead me to 

 separate definitely T. kraussi from T. ventricosa. It may well be the 

 case that we are here dealing with two branches of a not far re- 

 moved ancestral stock, both of which have evolved along similar 

 lines, but one more rapidly than the other. The more advanced 

 type may be recognised in T. kraussi, from the fact that it exhibits 

 all the late adult characters of T. ventricosa while yet in the early 

 adult stage itself ; the late adult characters of T. kraussi are never 

 reached in T. ventricosa. It is highly improbable, indeed, that this 

 giant form merely represents extreme individual variation in T. 

 ventricosa, and its separation from this seems to me to be 

 justified. 



There is a close general agreement with T. subventricosa 

 Stanton,* especially in the large dimensions, but the Patagonian 

 shell is relatively more elongated, has the valves less strongly 

 inflated, and the flattened frontal face less extensively developed. 

 In T. kraussi the umbonal region is more narrowed in form and 

 considerably more prominent, taking Dr. Stanton's figure to repre- 

 sent a typical specimen of the Patagonian shell. 



Both T. ventricosa and T. kraussi are figured by Krauss as repre- 

 sentatives of the same form, and he makes no mention of the 

 differences to which I have above alluded, though these are 

 plainly shown in the figures which accompany his description of 

 T. ventricosa. In deciding which of Krauss's specimens are to 

 bear the name T. ventricosa it is well to note that his descrip- 

 tion more accurately applies to the small specimens, and it 

 may further be observed that when he spoke of the species as 

 "particularly numerously" represented he must have referred 

 to the smaller form. This alone has been abundantly collected 

 and it has also been frequently spoken of in the literature 

 under the name T. ventricosa. Moreover, no less than six 

 authors have figured the smaller shells under the name given 

 by Krauss. 



* Stanton (3), p. 18, pi. iv., figs. 19, 20 



