126 Annals of the South African Museum. 



same features in T. tennis, from the Oomia beds in Cutch.* This, 

 however, is strongly distinguished from T. tatei by the very crowded 

 and delicate costate ornamentation of the flank, and by the slight 

 convexity of the valve. 



In view of the comparative rarity of the Costatae in the Cretaceous 

 rocks, the occurrence of so typical a member of the section in the 

 Uitenhage Series might appear at first sight to lend some support 

 to the view of those who have maintained that a part, at least, of 

 these beds must be ascribed to the Upper Jurassic. It is clear, 

 however, that in any attempt to uphold such a conclusion, less 

 weight can now be attached to the presence of Costatae than when 

 our knowledge of the distribution of this division of the Trigonia 

 was founded principally upon European occurrences. In Europe 

 itself, representatives of the section are sparsely present in the 

 Cretaceous, and these mostly show deviations which at once dis- 

 tinguish them from the typical Jurassic forms. Trigonia peninsularis 

 Coquand,f from the Aptian of Spain, exhibits a marked degeneration 

 of characters in its later growth-stages, while T. carinata Agassiz,J 

 from the Neocomian and Aptian, which may be brought into the 

 most intimate connection with the Costatae, shows equally far- 

 reaching modifications, although an examination of Lycett's figured 

 specimens and other well-preserved individuals fails to reveal any 

 characters which can be taken to justify Lycett's separation of this 

 form under a separate sectional heading. The typical features of 

 the section Costatae are exemplified, however, in a small Trigonia 

 described by E. Ascher from the Hauterivian of Silesia. || 



Although these European forms may be looked upon as lingering 

 representatives of a section which had passed its maximum develop- 

 ment, we must look further afield in order to complete the history of 

 this strongly characterised and long-lived division of the genus. In 

 the marine Oomia beds of Cutch, which may now be regarded as 

 of Lower Cretaceous rather than of Upper Jurassic age, aberrant 

 derivatives of the Costatae provide a striking feature in the molluscan 

 fauna ; but here also have been found two representatives, T. tennis 

 Kitchin and T. parva Kitchin, which retain in perfect manner the 

 typical characters of the section. South America has also furnished 

 another unmodified Cretaceous costate form in T. anguste-costata, 

 described by Behrendsen IT from strata which he regards as Upper 



* Kitchin (1), p. 35, pi. iii., figs. 5, 6. f Coquand (1), p. 129, pi. xxiii., fig. 3. 



} Agassiz (1), p. 43, Tab. vii., figs. 7-10 ; Lycett (3), p. 179, pi. xxxv., figs 3-6, 

 (1877). See also CoUot(l). 



|| Ascher (1), p. 159 [25], pi. xiii. [ii.], fig. 10. IT Behrendsen (1), p. 6, pi. 3, fig. 7. 



