The Relationship of the Tetracoralla to the Hexacoralla. 189 



4. Essential columella (columella propria). A separate unit 

 of structure continuous from early stages to old age. It may 

 be styliform, fasciculate, or lamelliform. 



Classes i, 2, and a part of class 3 fall into one group because 

 the structure of the central column is always connected with 

 structures outside of the central pit. Class 4 and the remainder 

 of class 3 contain those forms which are independent central 

 structures that do not involve any of the skeleton outside of the 

 central pit. In order to express this difference Gregory (1900) 

 has extended the term "parietal columella" to include classes 

 I, 2, and a part of class 3 of Milne-Edwards. Accepting this 

 change the terms will be used in the following sense in the 

 present paper: 



1. Essential columella (true columella). One that develops 

 independently of other calicular structures. It may be styliform, 

 lamellar, or fascicular. It is usually compact or solid. (See Fig. 

 6c, page 193.) 



2. Parietal columella (false columella, septal columella, 

 pseudo-columella). One that is formed by a specialization of 

 one or more septa or septal appendages. Such specializations 

 are: 



a. Enlargement of the inner end of a single septum. (See 

 Fig. 6h, page 193.) 



b. A central twisting of the inner septal ends. 



c. A development of large pali. 



d. The secondary deposit of calcium carbonate which usually 

 obscures somewhat the origin in cases b and c. 



A parietal columella may be compact or spongiose, fasciculate 

 ("paliform"), styliform or lamellar. 



Although forms with columellas appear as early as the Mid- 

 dle Ordovician (Black River) in Lindstrcemia whiteavesi, the 

 time of their greatest development is the Lower Carboniferous 

 (Mississippian), and corals with columellas have been an impor- 

 tant element ever since. While they have thus become more 

 nimierous in the later development of corals, suggesting that 

 such a structure is of some decided advantage, it is not obvious 

 what particular importance it could have had in the physiology 

 of the polyps. As a part of the stony calyx, the essential col- 

 umella is a sub-structure secreted by the basal disk. If its 



