The Invertebrate Fauna of the Uitenhage Series. 107 



quadratae, to receive Trigonia herzogi (Goldf.), the well-known 

 Uitenhage form, and T. transitoria Steinmann, from Lower 

 Cretaceous strata in Chili, Bolivia, and the Argentine Republic.* 

 T. neuquensis Burckhardt,! which is found associated with T. transi- 

 toria in the Neocomian Trigonia-beds of Las Lajas (Bio Agrio, 

 Argentine Eepublic), also falls within this division, and another 

 example of the same group is T. mamillata, which occurs in the 

 Oomia Trigonia-beds near Goonaree in Cutch.J These five members 

 of the Pseudo-quadratae show similar deviation from the characters of 

 the two sections with which they may best be compared. Like the 

 Quadratae, they have the escutcheon well ornamented, and the 

 sculpture of the area is in some respects very similar, especially in 

 the manner in which transverse ridges, particularly in the late adult 

 stage, may pass over to the escutcheon on the one hand and the 

 flank on the other. These characters contribute largely to distin- 

 guish the group from the Clavellatae, with which, both as regards 

 the youthful ornamentation and the adult form, it has much in 

 common. In the Quadratae the area is usually divided by its longi- 

 tudinal groove into a broader, superior, and narrower inferior 

 portion ; in the Pseudo-quadratae the upper division is the narrower. 

 A row of impressions or pits on the inner valve surface, near the 

 pallial margin towards the posterior end, is present in the Quadratae 

 but absent in the Pseudo-quadratae. In accordance with the position 

 of the longitudinal groove of the area, the lower siphonal channel on 

 the inner valve surface is relatively broad in the Pseudo-quadratae, 

 and may be almost twice as broad as the upper one. 



T. holubi is most closely comparable with the Indian T. mamillata, 

 to which it shows a striking similarity. In T. holubi, however, the 

 valve has a rather less convex form, the longitudinal groove of 

 the area is more strongly marked, and the tubercles of the ribs 

 in the early and middle adult stages are much less closely spaced. 

 The most important point of distinction lies in the development of 

 the sculpture of the area. In T. mamillata the phase in which 

 irregular transverse costae crossing the area form a dominant feature 

 is reached at a much earlier stage of growth than in T. holubi. 

 Thus, in T. mamillata, strongly developed costae appear on the area 

 soon after the neanic stage is passed and long before half the adult 

 dimensions are attained. In T. holubi, save for the longitudinal 

 groove and its accompanying line of tubercles, the surface of the 

 area is smooth, and only marked by lines of growth, until at least 



* Steinmann (2). f Burckhardt (2), p. 74, Taf. xiv., figs. 4-6. 



J Kitchin (1), p. 100, pi. ix., figs. 8, 9; pi. x., figs. 1-3. 



