118 Annals of the South African Museum. 



smooth, in other cases the ribbing extends to the frontal margin 

 at all parts. Near the pallial margin of large individuals senile 

 characters may be seen in the cessation of sculpture and the 

 appearance of several strong furrows of accretion. 



Two right valves in the specimen numbered 90h have a more 

 rounded and less sharply pointed anterior profile than any of the 

 other specimens studied. They also have the urn bones rather more 

 anteriorly placed. The other extreme, illustrating the less inequi- 

 lateral form of the valves and the more marked pointing of the 

 anterior profile, is shown by an individual from the collection of 

 the South African Museum. Characters quite intermediate between 

 these extremes are exhibited, for example, by a specimen numbered 

 12006 in the collection of the Geological Society of London. I 

 believe, therefore, that the material examined only suffices for the 

 establishment of a single species. 



Stow appears to have believed this form _ to represent T. van 

 Sharpe. \Vhen dealing with the Triyonia of the Oomia beds of 

 Cutch, I made reference to T. stoivi as an undescribed form allied to 

 T. van.- 1 ' In addition to the specimens there referred to, I have 

 since had the opportunity of examining further material in the col- 

 lections of the Geological Society and the British Museum (Natural 

 History), and also the fine specimens sent to me from South Africa. 

 T. stowi is a very well characterised form, peculiar for its elongated 

 outline, its pointed anterior extremity and greatly produced posterior 

 region, its striking flank-sculpture and its siphonal gape. Although 

 in many points, and particularly in the sculpture of the adult, it very 

 strongly recalls the Oomia T. v-scripta Kitchin, yet its youthful 

 characters plainly reveal its close relationship to T. van Sharpe (see 

 above). Until the young shell has attained a height of about 10 mm., 

 these two African forms are so alike that I have been unable to find 

 any feature by which they may with certainty be distinguished. In 

 the adult stage, however, T. stoivi has a more elongated outline and 

 is more strongly produced posteriorly ; its umbones are relatively 

 further removed from the anterior extremity ; the upper and lower 

 valve-margins converge towards the front to join in more or less 

 pointed form, and produce a frontal profile which stands in contrast 

 to the sweeping convex outline of the frontal margin in T. vau. The 

 angularly bent and swollen anterior portions of the ribs in the frontal 

 series, which produce such a peculiar pattern in the sculpture of the 

 anterior quarter of the flank in T. stoivi, are developed to a much 



Kitchin (1), pp. 66, 74. 



