320 



W. D. Lang — Jtirassic Polyzoa. 



majority of forms, however, sooner or later the Intermediate Type 

 of branching conies in, and in a great many forms this type is the^ 

 final one. In a few cases of Stomatopora, and in all Proboscina, 

 Type II is at some time or other reached, and remains the ultimate 

 form of branching of the zoarium. 



This sequence, namely, Type I — Intermediate Type — Type II, is 

 invariably followed. For, although an individual dichotomy may 

 occasionally occur of slightly more primitive order than its pre- 

 decessor, it is only an irregularity, and the general scheme of 

 development is in no wise obscured. 



In more primitive forms this evolution in branching does not 

 progress beyond Type I with a small angle. 



In the commoner forms of Stomatopora the ultimate branches are 

 formed on the Intermediate Type and on Type 11. 



Diagram I. — Curves showixg ttie Frequexcy of Braxchixg ix Stomatopora. 



Number of dichotomies from the first zocccium. 



In a few forms of Stomatopora (e.g. B.M. No. B. 4,822) Type II 

 oc<;urs after a few dichotomies, while it comes in very soon in some 

 primitive Proboscina (e.g. Proboscina Cnnningtoni, Gregory, B.M. 

 No. 97,617). 



In typical forms of Proboscina the early stages have been so 

 condensed, according to the law of acceleration (Tachygenesis),^ that 

 the first dichotomj' is formed on Type II (Fig. 10). Sometimes 



' A. Hyatt, " Bioplastology ami 'the related branches of Biologic research "r 

 Proc. Boston Soc. Nat. Hist.', 'vol. xxvi (1893), p. 77. 



