W. D. Lang — Jurassic Pohjzoa. 



317 



colonial stages he terms nepiastic, neanastic, epliebastic, and 

 gerontaatic, formed from the stem of the first terms suffixed with 

 the termination -astic, from to ila-rv, ' the city.' 



Cumings also terms the first-formed zoarium, which has hitherto 

 been known as the ' primitive disc ' in Cyclostomata and the 

 ' ancestrula ' in Cheilostomata, the ' protoeciv^m,' analogous to the 

 ' protegulum ' and ' protoconch ' in Brachiopods and Ammonites 

 respectively. 



Among the Jurassic forms of Stomatopora and Prohoscina, it has 

 been found that when any given character, such, for instance, as- 

 the ratio of the length of the zooecium to its breadth, is followed 

 from the first zooecium until the last, that it has a progressive 

 development, or anagenesis, reaches a maximum or acme, and often 

 may be seen to have a retrogressive development, or katagenesis, 

 in the ultimate branches of the zoarium. 



To illustrate this point, some examples of the character mentioned, 

 namely, the ratio of the length of the zooecium to its breadth, are 

 given below, the points of dichotomy of the zoarium being taken 

 as fixed points, and referred to by the numbers 1, 2, 3, etc., No. 1. 

 being the point in the zoarium marked by the first dichotomy, and 

 so on. The numbers with the names are the British Museum 

 register numbers of the specimens. 



The numbers in the above table, representing the length of the 

 zocBcium (the breadth being taken as 1), are, of course, averages ; 

 for at each dichotomy are three zooecia ; and if n is the number 

 of the dichotomy, the theoretical number of the zooecia of which the 

 average is taken will be 3 (2'*"^). Practically, however, the number 

 is smaller, owing to the loss of certain branches. 



The specimens whose zooecial lengths are given in the table are 

 chosen because they illustrate so well the regular changes of this- 

 character. Other specimens are more irregular, but all show to 

 some extent a definite plan of development. In the first four cases 

 given it will be seen that this chai'acter is anagenetic at first, and- 



