The Invertebrate Fauna of the Uitenhage Series. 103 



T. transitoria, which occurs in Neocomian strata in Bolivia, Chili, 

 and the Argentine Kepublic. T. transitoria shows a closer approxi- 

 mation of characters to the Clavellatse ; it is not so elongated in 

 outline, and its strongly nodose ribs have a curved form and a forward 

 sweep when traced downwards, throughout almost the whole extent 

 of the flank. In T. transitoria the rows of nodes representing 

 the marginal and median carinse become joined by transverse rounded 

 ridges at an earlier growth-stage than in T. herzogi, in which these 

 rows of nodes, and that representing the inner carina, remain 

 isolated and distinctly developed until traced farther back towards 

 the siphonal margin of a fully grown individual. The escutcheon 

 is smooth in its anterior part in T. transitoria, w r hile ornamented at 

 an early stage in the African shell ; in T. transitoria the escutcheon, 

 generally speaking, is more sparsely sculptured. 



I have elsewhere pointed out the features of distinction between 

 T. herzogi and T. mamillata, from the Oomia beds of Cutch.* 

 T. mamillata shares the principal characters of thePseudo-quadratse, 

 but is a shorter shell than T. herzogi, and is vertically truncated 

 in front. In T. mamillata those ribs situated at the centre of the 

 flank, and posteriorly to this, are slightly curved, and are directed 

 anteriorly at their lower ends, while the corresponding ribs of 

 T. herzogi are backwardly directed and are not curved. On the 

 area of T. mamillata, the rows of more or less isolated nodes become 

 united to form smooth transverse ribs nearer to the umbo than in 

 the African form, in which, also, the lines of nodes and transverse 

 ridges of the area are rather more closely spaced. In T. mamillata 

 the tubercles are more closely crowded together in the ribs of the 

 flank and are, generally speaking, more robust and conspicuous than in 

 T. herzogi, particularly in relation to the size of the valve. The blunt 

 transverse ridges of the area towards the posterior end of an adult 

 specimen are broader and more strongly developed than in T. herzogi. 



A comparison with Trigonia holubi is given in the remarks which 

 follow the description of that form. 



Two specimens depicted in the left-hand side of a text-figure 

 published by Drs. Hatch and Corstorphine t represent T. herzogi, 

 but very greatly reduced in size. 



TRIGONIA HOLUBI sp. nov. 



Plate IV., figs. 2, 2a. 



Description. The shell is large and massive, almost oblong in 

 outline, and usually vertically truncated in front, or with very steep 

 * Kitchin (1), p. 102. f Hatch and Corstorphine (1), p. 245, fig. 66. 



