42 Annals of the South African Museum. 



similarity in the adult stage in members of these two groups, 

 however peculiar and striking may be the characters which appear 

 to unite them, and at the same time to differentiate them from all 

 other sections of the genus with which we are acquainted. Since 

 the members of these two groups appear to illustrate mere homceo- 

 morphy, their value as evidence in the correlation of the faunas 

 becomes very much reduced, and if dissociated from the forms which 

 accompany them, they could not well be considered to afford proof 

 of contemporaneity. But whatever be the causes that determine the 

 evolution along converging lines in shells which, by their youthful 

 characters, betray a heterogenetic origin, we may in this case safely 

 infer from their occurrence with an association of forms in so many 

 respects similar, that they acquired their common characters at 

 approximately the same time. 



Before concluding this comparison of the Uitenhage and Oomia 

 Trigonice, attention may be directed to certain broad features of 

 general habit which in some measure lend a distinctive aspect to 

 several members of the genus in the faunas under discussion ; and 

 it will be noticed that while these features serve in great degree to 

 imprint a facies on the assemblage which brings it into contrast 

 with European occurrences, the same broad distinguishing characters 

 are not confined to one section of the genus, but are shared by 

 members of stocks not intimately related. There is the tendency to 

 great posterior elongation of the shells, and in some cases a 

 siphonal gape ; the obliteration of the carinae with disappearance of 

 a definite demarcation between flank, area, and escutcheon ; the 

 dwindling and disappearance of sculpture on the area ; and in 

 several instances the situation of the urnbones relatively far from 

 the anterior extremity. In the Oomia beds these points are illus- 

 trated in varying degree in certain degenerate derivatives of 

 Costatae, and in the group of T. v-scripta ; in the Uitenhage beds 

 they are exemplified in T. vau, T. stowi, T. rogersi, and T. conocardii- 

 formis. In both Trigonia vau and T. dubia the parallelism with the 

 genus Gomomya cannot be overlooked, and I have previously 

 suggested that if complete shells of these could be procured, they 

 would be found to gape at the siphonal end ; this idea is now 

 supported by a specimen of T. stoivi sent to me from the South 

 African Museum, which is almost uninjured at the siphonal border, 

 and which plainly exhibits a gaping habit. 



We do not find further aids to comparison amongst the few Oomia 

 Mollusca which have already been described, and a detailed account 

 of the remaining lamellibranchs collected by Wynne and Stoliczka 



