W. D. Lang — Jurassic Polyzoa. 319 



tlie author hopes to consider in a future paper ; all that is wished 

 at present is to demonstrate the importance of following each 

 character through its own development in the zoarium, and by this 

 means determining its value as an index to the relationship 

 of one zoarium to another. The fact that each character has a 

 developmental history makes it clear that the diagnosis of a form 

 is incomplete, and for practical purposes useless, unless the part 

 of the zoarium with respect to its age is specified. 



The last point to be considered is the method of branching in 

 the Jurassic forms of the two ' genera ' Stomatopora and Prohoscina. 



In a single series of zooecia, such as is typical in the genus 

 Stomatopora, two ways of branching may be noticed, namely, lateral 

 branching and dichotomy. 



In lateral branching a new zooecium arises from any point in 

 a chain of old zooecia, and generally diverges at a wide angle (see 

 Diagram II, Fig. 1, p. 321). It is common in Silurian and Cretaceous 

 forms, but has not been observed (except in one doubtful case) in 

 any Jurassic form. 



In dichotomy, which always occurs in Jurassic forms, 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 II, Figs. 2-9, p. 321.) 



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 from one another throughout 

 their whole length (Diagram II, Figs. 2, 3, 9, p. 321), only touching 

 at their bases. In Type II 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). 



To a large extent correlated with the type of branching is the 

 angle of divergence of the two new branches. This angle tends 

 to diminish distally. But that it is not wholly dependent upon 

 the type of branching may be seen in cases where the new branches 

 diverge at an angle of as much as 60° after branching according 

 to Type II (Fig. 5), while in other cases (Fig. 8) the two new 

 branches may remain contiguous until they branch again. In the 

 majority of cases, however, the angle of divergence and the type 

 of branching are so closely correlated that for practical purposes 

 they may be considered together. 



Starting from the first zooecium, which arises directly from the 

 primitive disc, one or two zooecia generally follow before the first 

 dichotomy takes place. This in all Stomatopora and in a few 

 Prohoscina (e.g. P. Cunningtoni, Gregory, B.M. No. 23,852, zoarium. 

 marked 1) is after Type I with a wide angle nearly always 180° 

 (Fig. 9). The second dichotomy in the great majority of cases is 

 on Type I, with an angle of divergence of 120°. The angle of the 

 next dichotomy is commonly 90°, of the next 60°, of the next 45°, 

 Type I being still the mode of dichotomy. 



In primitive forms (e.g. S. Waltoni, Haime, B.M. No. B. 2,287) 

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



