NORTH AMERICAN EARLY TERTIARY BRYOZOA. 
653 
Iu dichotomy, two new zooecia arise from the end of an older zooecium; the angle at 
which they diverge varying from 180°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 
Fig. 215.— Forma Stomatopora Bronn, 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 Proboscma. 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; 6, intermediate type with small angle; 7, 
intermediate types with large angle; 8, type II with an angle of 0° ; 9, type I with an angle of 
180° ; 10, the first dichotomy is after type II with an angle of 0° ; 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, 8) ; 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). 
