440 STUDIES IN EVOLUTION 



presents a more arcuate first septum, and that the second is 

 comparable with the third or fourth of T. uniangulare, clearly- 

 indicating that the development has been accelerated by the 

 skipping of at least two phases of growth in the septa. In 

 other characters the two forms merely indicate differences 

 which are probably only of specific importance, such as the 

 more angular form of the lobes and saddles in T. retrorsum, 

 var. tyjyum, and the absence of the minute cone at the summit 

 of the annular lobe. 



