•318 W. D. Lang — Jurassic Vohjzoa 



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

 The fifth example reaches its acme at 2, while the sixth is at its 

 acme at the first dichotomy, and decliues after the second. 



As far as actual length is concerned, numbers 2, 4, 5, and 6 are 

 practically the same, while No. 1 has much longer and No. 3 much 

 shorter zooecia than the rest. 



This particular character was chosen onl}' as an example. Other 

 characters show a similar regularity in development, according 

 to the part of the zoarium in which they are situated. Tliose which 

 have been observed and treated in the same way as the length of 

 the zooecium are four, two zoarial and two zocecial. The zoarial 

 are the method of branching, which will be treated later, and the 

 frequency of branching, which is measured by the number of 

 peristomes between two dichotomies. The zocecial are the shape 

 of the zooecium, and its ornamentation by transvei'se ribs. 



Two characters which have been used by former workers have 

 been found by the author so unpractical that they have been given 

 up as useless ; these are the height of the peristome and the punctation 

 of the zooecium. 



The first of these, though doubtless an excellent character where 

 the state of preservation of the fossil is such that its presence may 

 be counted on, becomes useless in the fossils here dealt with, because 

 in the majority of cases the whole peristome has been broken off, 

 leaving it impossible to say whether this structure was high or low 

 when the organism was alive. 



Again, the appearance of the punctation of the zooecium seems 

 to depend to such a large extent upon the state of preservation 

 of the zocecial wall, that its presence is of little use for systematic 

 work. Nor does it appear to show any variation during the growth 

 of the zoarium. 



The results obtained from the study of the development of the 

 characters previously mentioned, namely, the frequency of branching, 

 the shape of the zooecium, and the transverse ribbing of the zooecium, 

 show that the rule in the majority of cases is as follows : — 



1. Freqiiencij of hrancliing. — The number of peristomes between 

 the first two or three dichotomies is small (nearly always 1 or 2), 

 then suddenly increases largely, and finally becomes small again. 



2. Shape of zocechim. — Generally the zooecia are either cylindrical 

 or pyriform. In many of those forms which have cylindrical 

 zoo3Gia throughout the greater part of the zoarium, the zooecia 

 between the first and third dichotomies tend to be slightly pyriform ; 

 ■while in those forms with pyriform zooecia, the zooecia between 

 the first and third dichotomies are generally more pyriform than 

 the rest. 



3. Transverse ribbing of the zooecium. — Eibbing, when present, 

 is usually faint at its first appearance, becoming stronger later on, 

 and in some cases becoming fainter again finally. The point at 

 ■which the acme is reached varies a great deal. 



The systematic value of any one of these characters and the 

 amount that the consideration of them affects the question of species 



