The Relationship of the TctracoraUa to tJie HexacoraUa. 191 



already formed often obscures the mode of formation, a diffi- 

 culty that is met with in examining the adult forms of 5^. radians. 

 Ogilvie thought the term "paliform pseudo-columella" to be 

 applicable, and it was only from a detailed morphological study 

 of all stages in the development that the columella of 6". I'adians 

 could be decided definitely to be an essential columella. In 

 transverse section it is most often round, but sometimes is oval, 

 with its long diameter in the direction of the directive mesenteries. 

 Cyathaxonia. — Among Paleozoic forms, Cyathaxonla has an 

 essential columella, the developmental stages of which are indi- 

 cated by Carruthers (1913) and may be briefly summarized as 

 follows : 



1. The columella is seen first at the time when eight or nine 

 septa have appeared. The first septa meet at the center without 

 any noticeable axial thickening. 



2. The columella quickly attains prominence as a structure 

 independent of the septa and tabulae. 



The beginning of the columella cannot be independent of the 

 septa in this case as the latter meet at the center, therefore there 

 is a stage in the life history of Cyathaxonia at which the 

 columella is parietal. 



Lophophyllum. — Lophophyllum has a parietal columella of the 

 lamellar type. It is never independent of the septum, from 

 which it is an outgrowth, although it sometimes becomes styliform 

 and projects as much as half an inch above the bottom of the 

 calyx. This stylif-orni aspect may be seen in Lophophyllum tor- 

 tuosum (= L. konincki Milne-Edwards and Haime). When 

 lamellar, its long axis lies in the direction of the cardinal and 

 counter septa. It is usually a continuation of the counter septum, 

 but sometimes the cardinal and other septa are involved. The 

 youngest stage figured by Carruthers shows twenty-seven septa 

 and has a well formed columella. 



Aulo phyllum. —SiaxAty Smith (1913) has described in detail 

 the morphology and ontogeny of the genus Aiilo phylhim. This 

 description clearly indicates the development of an essential 

 columella. The process is as follows : 



1. Six septa meet at the center. 



2. A zaphrentoid stage in which the septa meet and coalesce 

 at the center. 



