322 W. D. Lang — Jurassic Pohjzoa. 



a second dichotomy on Type 11 follows, but often the arrangement 

 of pei'istomes is quite irregular after the first dichotomy. 



In the most advanced types of Froboscina, e.g. P. Eudesi, Hairae, 

 B.M. No. D. 1,843, the arrangement of peristomes is irregular from 

 the first (Fig. 11) — the arrangement typical for Berenicea, the next 

 'genus' in the sei'ies of which Stomatopora and Froboscina are the 

 first two terms. 



The absolute regularity of the sequence of these different types 

 of branching and the condensation of the more primitive types in 

 the more advanced forms show of how much importance this 

 character is in determining the relationships of different forms. 

 Moreover, it is worthy of notice that, while in the Jurassic forms 

 of Stomatopora Type II is not very common, it becomes extremely 

 common in the Cretaceous forms, though the sequence in these is 

 considerably obscured by the superimposition of lateral branching 

 upon the dichotomy. 



In the case of the Cretaceous Stomatopora the lateral branching 

 seems to be the reappearance of a character which was formerly 

 present and has been lost, for it occurs in Silurian forms of Stomato- 

 pora, and is apparently absent in Jurassic forms. 



Taking the type of branching as a character of primary importance, 

 and following this and the other characters in their development 

 from the beginning of the colony, series can be traced and natural 

 relationships established. The true genera will probably be found 

 to correspond to some extent with the present ' species.' But before 

 this can be done at all satisfactorily it will be necessary to work 

 through a great deal more material, cai'efully collected according 

 to horizon and locality. 



It is easy to represent graphically the evolution of the characters 

 of two forms for comparison by means of curves. Diagram I, on 

 p. 320, gives an example. 



The writer of this paper, intending only to introduce his idea 

 and method of dealing with this difficult group, as a means of 

 establishing a natural classification, has purposely avoided entering 

 into much detail, and confined himself rather to general statements. 

 But illustrations are taken from actual specimens, and these may 

 be seen in the British Museum. "What is needed is more material 

 which shall test the above propositions. What has been attempted 

 is not arbitrarily to select a character and invest it with specific 

 or generic importance, but by tracing the development of the 

 character to assign it to its appropriate rank. The terms genus 

 and species can then be applied with some meaning, and new 

 forms, as they occur, will fall into their proper places in a natural 

 scheme. 



