94 Annals of the South African Museum. 



perhaps be closely related. It appears, however, to have more 

 closely spaced and less coarsely nodose ribs. To judge from the 

 description and figures of T. delafossei given by Paulcke,, the 

 escutcheon is more sharply demarcated from the area than in 

 T. ventricosa; T. delafossei also does not appear to have attained 

 such large dimensions. Another South American form, T. nepos 

 Paulcke, from the Neocomian of Chili, shows to some extent a 

 similarity to T. ventricosa, especially if the smaller individuals 

 figured by Paulcke * be compared with specimens of similar size ; 

 T. nepos, however, has more delicate sculpture and is also well 

 distinguished by the peculiar arrangement of the ribs towards the 

 frontal face. In the work cited above, Paulcke brings T. nepos and 

 T. delafossei into close relationship ; yet he perceives in T. nepos an 

 exceedingly close resemblance to T. baylei Dollfus, t from the Kime- 

 ridge of Europe, and recognises in the similarity an indication of the 

 scaphoid ancestry of these Cretaceous forms. It may be pointed 

 out that the principal character upon which he relies, the existence 

 of a more or less independent series of anterior ribs, is a feature to 

 which, alone, such significance can certainly not be attached. It 

 represents a plan of sculpture which has appeared repeatedly and 

 independently in various Trigonia- stocks, and in T. baylei and other 

 typical Jurassic Scaphoideae is associated with well-marked cha- 

 racteristics which indicate the very high improbability of such 

 direct relationship as that suggested. In T. ventricosa, T. delafossei, 

 and T. nepos, just as in the T. aliformis group of Europe, the very 

 prominent and highly inflated umbonal region, the extreme incurva- 

 tion of the umbones, the relatively very narrow area and the 

 crenulation of the shell margin, are characters which are conspicu- 

 ously developed in these well-marked and specialised groups. In 

 T. baylei and similar Scaphoideae, the laterally compressed form, 

 the weak umbonal incurvation, the nodose carinae and delicately 

 sculptured area, no less than the relations of frontal to lateral 

 costae, are also characters of specialisation, and judging by the 

 evidence of early growth-stages, these shells had ancestry in 

 simple clavellate forms. A similar though independent ancestry 

 seems also very probable in the case of the various groups classed 

 under the broad heading Scabrae. 



In the published description of the Trigonia from the Oomia 

 Group in Cutch, it is stated that the Indian specimens ascribed to 

 T. ventricosa " although exhibiting great variability, offer no features 



Paulcke (1), p. 293, Taf. xvii., fig. 8. 



f Dollfus (1), pi. xv. ; Bigot (1), p. 309 [51], pi. xii., fig. 10. 



