NORTH AMERICAN EARLY TERTIARY BRYOZOA. 



653 



In dichotomy, two new 'zooecia arise from the end of an older zooeciuui; the angle at 

 which the.v diverge varying from ISO to 20 or 30, and varying in a definite manner (see 

 diagram 2, figs. 2-9). 



Dichotomy in the forms under consideration occurs in three types, one of which is 

 intermediate between the other two. In that termed type I the two new zooecia are separate 



B 

 FIG. 215. Forma Stomatopora Bronu, 1S25. 



A. Diagram to show the changes in the method of branching and in the shape of the zooecium 

 in Stomatopora antiqua Haime and its related Liassic forms. B. Diagram showing the method 

 of branching in Jurassic forms of Stomatopora and Proboscina. 1, lateral branching; 2, type I 

 with large angle ; 3, type I with small angle ; 4, type II with small angle ; 5, type II with large 

 angle preceded by intermediate type with large angle; G, intermediate type with small angle; 7, 

 intermediate types with large angle; S, type II with an angle of ; 9, type I with an angle of 

 180 ; 10, the first dichotomy is after type II with an angle of ; 11, the arrangement of the 

 peristomes is irregular from the first. 



from one another throughout their whole length (diagram 2, figs. 2, 3, 9), only touching at 

 their bases. In type 2 they are contiguous throughout their length (figs. 4, 5, S) ; and they 

 are contiguous for part of their length in the intermediate type (figs. 5, 6, 7). The angle 

 of divergence of the two new branches tends to diminish distally. 



The frequency of branching is measured by the number of peristomes between two 

 dichotomies. The number of peristomes between the first two or three dichotomies Is small 

 (nearly always one or two), then suddenly increases largely (anagenesis) and finally becomes 

 small again (katagenesis). 



The ratio of length of zooecium to breadth is progressive (anagenetic) at first and 

 reaches its acme at the third dichotomy, after which it is retrogressive (katagenetic). 



