Tlie Invertebrate Fauna of the Uitenhage Series. 109 



of the tubercles of the flank costae they become crowded closely 

 together on a given rib is a very striking feature. In T. mamillata 

 from Cutch, these characters are also coincident, but they are 

 acquired at a much earlier period of growth. The members of the 

 Pseudo-quadratye, in the three continents where they are known 

 to occur, seem to have been destined to pass through a similar 

 sequence of developmental phases, but these phases were not 

 reached by all the species at the same time. Thus, T. mamillata 

 is in advance of T. holubi. In T. holubi, the stage where the area 

 is demarcated by an upper and lower marginal row of tubercles and 

 marked by a median longitudinal row, persists until half the adult 

 dimensions have been reached. In T. mamillata, this trituberculate 

 stage is passed over rapidly and is superseded in very early adult 

 life. In T. herzogi it is superseded by the transversely costate stage 

 (the area alone is still referred to) at a somewhat later period of 

 development, but still not so late as in T. holubi. A point of great 

 interest is that in T. mamillata and T. herzogi, and also in 

 T. transitoria, the trituberculate stage may be seen to pass into 

 the transversely costate stage by the progressive transverse elonga- 

 tion of the tubercles in the three sets, and the gradual coalescence 

 of the transverse ridges thus formed. In T. holubi, this inter- 

 mediate phase of development is not so noticeable : it is very much 

 suppressed or is wholly omitted. 



The facts here briefly set forth seem to suggest that the group 

 Pseudo-quadratae may be of an artificial character to some extent 

 that the forms here included for convenience of classification are not 

 strictly homogenetic ; and many known parallel instances amongst 

 molluscs and brachiopods give great probability to the truth of this 

 idea. We must suppose all these forms to have been descended from 

 true Clavellatae, just as in the case of the Quadratee of Europe, and 

 it is a very striking circumstance that the characters which mark 

 the Pseudo-quadratae should appear approximately at the same 

 geological moment of time in three widely remote continents. 

 Yet these forms are unknown in the European area. The Post- 

 jurassic development shown by the Quadratae and Pseudo-quadratae, 

 each group representing offshoots from one or more clavellate stocks, 

 took place along parallel though independent lines in the European 

 area and in the southern development with which we are dealing. 

 Within each area, again, the development in the supposed several 

 genetic series (which were departing along the same general lines 

 ftom the ancestral clavellate characters) may be supposed to have 

 been in some degree independent. 



