332 



be by one or other of three types as folloAvs : In type I the two zoceeia are 

 separate throughout their entire length, only touching at their bases. In 

 type II they are contiguous throughout their length, and in the interme- 

 diate type they are contiguous for part of their length. To a large extent 

 correlated with these types of dichotomy is the angle of divergence of the 

 branches. 



In all the Jurassic stomatoporas and in a few proboscinas the first 

 dichotomy is according to t3qje I, and at a very wide angle (1S0°). The 

 second dichotomy, in the majority of cases, is also according to type I, 

 with an angle of 120°. The next is commonly only 90°, the next 60°, and 

 the next 45°. all according to type I. "In primitive [Jurassic] forms the 

 branching never gets beyond type I with a small angle. In the majority 

 of forms, however, sooner or later the intermediate type of branching comes 

 in. and in a great many forms this type is the final one. In a few cases of 

 Stomatopora. and in all Probosciiia, 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." (35). 



In primitive Proboscina (a genus derived from Stomatopora) the first 

 dichotomies are according to type 1. "In the typical forms of Proboscina 

 the early stages have been so condensed according to the law of accelera- 

 tion (Tachygenesis). that the first dichotomy is formed on type II. 

 . . . . In the more advanced types of Proboscina .... the ar- 

 rangement of peristomes is irregular from the first." This is the typical 

 arrangement for Bernicea, a derived genus of which Stomatopora and 

 Proboscina are the first two terms. It is worthy of notice that while in 

 the Jurassic forms of Stomatopora type II is not very common, it is ex- 

 tremely common in the Cretaceous forms. 1 



Graptolites. — The beautiful researches of Ruedemann in this group 

 have shown us. as pointed out above, that the graptolite colony closely 

 approaches the morphologic value of an individual, and that, like the in- 

 dividual, it presents definite ontogenetic (astogenetic) stages. Ruedemann 

 C47) applies to the colonial development the terminology proposed by 



1 For studies in the zoarial development of Bryozoa see Curnings, E. R.. The 

 development of some Paleozoic Bryozoa, Am. Jour. Sci., vol. xvii. 1904 ; Develop- 

 ment of Fenestella, Am. Jour. Sci., vol xx, 1905. Lang, W. D.. The Jurassic 

 forms of the 'genera' Stomatopora and Proboscina, Geo}. Mag., Dec. v, vol. i, 1904 ; 

 The Reptant Eleid Polyzoa, Geo]. Mag. Dec. v, vol. Hi, 1906 ; Stomatopora antiqua. 

 Haime. and its related Liassic forms, Geol. Mag., Dec. v. vol. ii. 1905. 



