108 Annals of tJic SoutJi African Museum. 



half the adult dimensions are attained. The transverse ribs which 

 are subsequently developed on the area are not so well defined or so 

 robust as in T. mamillata, 



From Trigonia lierzoyi (Goldf.),* T. holnbi is readily distinguished 

 by the considerably shorter form, the vertical anterior truncation, 

 the more crowded and much more prominent character of the flank 

 ornaments, and the later appearance of transverse sculpture on the 

 area. In the middle of the flank of an adult specimen, the ribs of 

 T. licrzoiji are straight, while those of T. lioliibi are curved. The 

 backward slope of the posterior flank ribs in T. licrzorji is another 

 point of distinction. 



There is a marked general similarity to Triamu'a transitoria 

 Steinmann,! from South America, but the ornamentation differs 

 considerably. In T. transitoria the tubercles of the costse on the 

 flank are placed closely against one another through all adult stages 

 instead of being openly spaced as in the ribs of T. liolubi. In T. tran- 

 sitoria also, the development of transverse ridges on the area comes 

 in at a much earlier stage of growth, and, in fact, this ornamentation 

 is a marked character during a large part of the adult life. There 

 appear to be other points of difference which it is unnecessary to 

 enumerate. 



T. nciiqucnsis Burckhardt is at once readily distinguished from 

 T. liolubi by its much shorter and higher outline. Here, also, the 

 transverse ridges of the area become a marked feature at an earlier 

 growth-stage than in T. liolubi. In T. neuquensis, the manner in 

 which these ridges in the posterior part of the shell extend down- 

 ward on to the flank is much more marked than in the most advanced 

 stage in T. liolubi. 



Note on the Development of tlie Pseudo-quadrate Trigoniae. 



A strong contrast between the features of the neanic and adult 

 stages may be observed very frequently in species belonging to 

 various sections of this widely conceived genus, but it is seldom that 

 a transition in characters so marked and abrupt as that shown in 

 T. liolubi is seen to occur during the adult period. The sudden 

 acquirement of strong transverse ridges on the area, a character 

 which is accompanied by a change in the manner of development 



* Goldfuss (1), Band ii., p. 202, Tab. 137, fig. 5 (1837). 



t Steinmann (1), p. 260, Taf. iii., fig. 3; Steinmann (2), p. 221, Taf. vii., 

 figs. 3, 4, Taf. viii., tigs. 1-3; Burckhardt (1), p. 21, pi. xxv., figs. 1-3; 

 Burckhardt (2). p. 73, Taf. xiv., figs. 1, 2. 



