MORPEIOLOGY OF THE CORAL SKELETON. 51 



there is in each iutei'septal loculus a mesentery (paries), l)ut still, in appa- 

 rent contradiction to von Koch, the theca is the so-called true theca (eutheca). 



From the great variation not only in the same species, but in a section 

 of a single corallite, no special systematic importance can be attached to 

 the theca being of the so-called true or false variety. True theca marks 

 the Anlagen of new septa, as vou Kocli and Bourne liave shown, or occurs 

 in calices where the septa are distant from one another and their outer ends 

 are not sufficientlv thickened to etfect peripheral fusion. True theca is 

 correlated with two factors in the gro\vth of the individual coral — the first 

 factor is simply a stage in the development of the septa, and the second 

 is merely that of sj^acial relation. The character of the theca will vAry 

 with the variabilit)' of these two factors In the case of the first, the 

 variability will depend upon the retardation t)r acceleration of the distal 

 and proximal (thecal) i)ortion of the young septum with reference to the 

 free inner portion, and in the second upon the distance between the septa. 

 These two factors may be to a large extent functions of each other. 



Before ending the discussion of the wall of the so-called " Madreporaria 

 Aporosa," a few words will be added on tlie wall of the Turbinolid corals. 

 Ogilvie,' in her definition of the famil}-, says: "A true theca or pseudotheca 

 is present peripherally; in all cases coalescent with the epitheca." On page 

 250 she says: "In many Turbinolid* the deposit of calcareous matter on 

 the outside of the thecal centers of calcification becomes very great, and may 

 be either compact or show hollow spaces." The character of the epitlieca 

 in Turbinolids is variable; in Paracyatlms bellus(see PI. VIIT,figs. IC to 18) it 

 consists merely of occasional encircling slu-eds, while in Flabellum it is closely 

 applied to the wall, often highly })olislied and porcellanous in nature. There 

 are many Turbinolids in which there is no vestige of an epitheca, e. g., 

 Platytrochus, Discotrochus, etc. I do not see more reason for calling the 

 external thickening of tlie wall and peripheral ends of the septa of such 

 corals as Paracyatlms alternatus (p. 10.5) ejntheca than I can for calling the 

 internal growth of tliese parts by the same naine. In corals possessing an 

 edge zone the peripheral parts of the skeleton should grow just as those 

 parts within the wall. If the ej)itheca is formed at the bend between the 

 aboral body wall and oral body wall, we must try at least to distinguish 



I Op. cit., p. 333. 



