106 Annals of the South African Museum. 



corroded by weathering. In these specimens the etching has attacked 

 the shell along lines of weakness, and has brought the tubercles and 

 ornamenting ridges into undue relief, which results in an appearance 

 so deceptive that for some time I was in doubt whether these etched 

 specimens should not be regarded as a distinct form. A careful 

 comparison between these individuals and others which have not 

 suffered from weathering shows that they agree in all essential 

 characters and that a separation cannot on present evidence be 

 established. In addition to differences due to preservation, there is 

 some variation in the sculpture of the area and escutcheon which 

 may be noted here. The transverse ornaments of the area are in 

 some individuals more robustly developed than in others ; on the 

 escutcheon the linear coalescence of tubercles to form parallel, 

 obliquely running ridges is more complete and regular in some 

 specimens than in others, and the ridges replace the isolated 

 tubercles at an earlier stage of growth. There is also some variation 

 in the arrangement of the tubercles in the lower half of the flank of 

 adult specimens near the frontal margin. The lower terminations 

 of the ribs here tend to turn forwards along the more prominent 

 ridges of growth ; some of the tubercles may appear to blend with 

 these ridges, which turn up sharply at the frontal face. Another 

 point in which some variation is shown is in the form of the anterior 

 profile of the shell. In some specimens there is vertical truncation 

 in front, in others the outline of the anterior margin slopes forward 

 somewhat, when traced down from the umbones, so that the foremost 

 point of the shell is situated near to the junction of the inferior and 

 anterior margins. 



This is the form referred to by Sharpe as a Trigonia allied to 

 T. herzogi ; he gave the following description of specimens which 

 were presented by J. S. Bowerbank to the Geological Society of 

 London : " Trigonia sp. (?). This variety (or possibly distinct 

 species) is closely allied to Trigonia herzogi, but is more quadrate 

 in outline than the common variety, the anterior edge being 

 truncate ; and the costal rows of knobs turn forward as they 

 approach the ventral border, instead of passing downwards and 

 backwards. These specimens came from Algoa Bay, and apparently 

 have been derived from the Sunday Eiver district, from the aspect 

 of their matrix." * 



T. holubi clearly cannot be united with either the Clavellatse or 

 the Quadratae, but its characters are those of the somewhat inter- 

 mediate kind which led Steinmann to establish the group Pseudo- 



* Sharpe (1), p. 202. 



