296 



Prof. J. W. Gregory. 



stage before the development of septa. The next figure (fig. 65) shows, 

 a slight increase in the size of the central tube and reduction in that 

 of the peripheral tubes. In the next stage (fig. 6c) the central tube is 

 large, and is surrounded by a zone of compressed tubuli. Finally, 

 there is the stage in which the septal structures appear. This stage is 

 illustrated (fig. 6^^) by a calicle with one well-developed septum, which 

 is the continuation of the wall separating two adjacent tubuli. 



That the calicles and septa of Heliolites are formed by the same pro- 

 cess appears probable from evidence cited by Lindstrom, who has given 

 a series of figures showing the development of a group of caeca into a 

 large calicle, some of the csecal walls remaining as septa (see Plate 2, 

 %• 7, a-g). 



Hence it appears probable that the septa of Heliolites are not homo- 

 logous Avith the septa of Madreporaria ; for they are the remnants of 

 walls and not special outgrowths from the margin of corallites. They 

 are as much " pseudosepta " as the corresponding ridges in Heliopora. 

 Why the septal structures are, as Professor Nioholson remarks, 

 " approximately constant " in number and large in Heliolites, while they 

 are small in size and variable in number in Heliopora, is easily 

 explained. It is, in fact, the necessary consequence of the difference 

 in size and regularity of the coenenchymal caeca in the two genera. 

 The cseca of Heliopora are relatively more numerous, smaller, and less 

 regular than in Heliolites. Accordingly, as the calicle of Heliopom grows 

 and absorbs the surrounding caeca, there is left a considerable and 

 variable number of septal ridges. 



That the caeca of the modern representatives of the Heliolitidae 

 should be smaller than those of the Palaeozoic forms is not surprising. 

 It is the natural line of development. Heliopora may therefore be 

 explained as a Heliolitid in which the caeca have decreased in size and 

 increased in number. 



C. The Calicular Them of Helioliticke. — According to Lindstrom, " the 

 feature which decidedly removes it [Heliopm-a] far from the Heliolitidae 

 is the total want of a calicular theca." Professor Freeh gives different 

 expression to the same idea ; he states [6, p. 500] that the walls of 

 the calicles in Heliopora are perforated and incomplete, whereas those 

 of Heliolites are complete, and the calicles are closed tubes. Freeh's 

 statement is, however, not correct, as a matter of fact \cf. Moseley, 13, 

 p. 112] ; but his idea is apparently the same as that which has been so 

 beautifully worked out by Professor Lindstrom. 



Heliolites, according to Lindstrom, has a true theca,"^ which is the 



* It may save some misunderstanding to remark that Lindstrom distinguishes 

 three thecal structures : (1) the calicular theca which bounds the inner axial pare 

 of the calicle ; (2) the external theca, which includes the calicular theca and all 

 the coenenchyma which has developed from it ; and (3) the coenotheca {non 

 Bourne), which covers the lower part of the corallum like the ejpitheca of com- 

 pound Madreporaria. 



