114 SYNOPTIC COLLECTION. 



these parts of the cells, owing to the accumulation in 

 them of food material. 



It is probable from the evidence now at hand that the 

 ancestral form above described, developed four mesen- 

 teries. The Hexactiniae, to be described farther on, pass 

 through a stage with four mesenteries, which antedates 

 the Edwardsia stage of eight mesenteries. This form 

 may have given rise to a branch which through continued 

 specialization reached the condition now shown in the 

 Alcyonaria. It is customary to consider the Alcyonaria 

 as more specialized than the Hexactiniae, and for this 

 reason they are generally placed after this group; but set- 

 ting aside their variety of form and delicacy of structure, 

 they seem in reality more simple, especially when the 

 single, generalized Alcyonarians are considered. 



There is also another good reason for placing the Alcyo- 

 naria as the more primitive group. Recent investiga- 

 tions 1 upon Alcyonaria and the ancient tabulate corals 

 tend to prove that the latter group (with the exception of 

 a few species) are ancestral forms of the Alcyonaria. 

 For this reason some of the tabulate corals have been 

 taken from the Zoantharia where hitherto they have been 

 placed, and are here considered as the primitive fore- 

 runners of the Alcyonaria. 



Cladochonus ( = Pyrgia) michelini M.-Ed. & H., is a 

 single, trumpet-shaped form when young (PI. 156, fig. i, 

 natural size; fig. 2, enlarged), and though without pores, 

 walls, or horizontal floors, called tabulae, it is probably 

 related to Aulopora (No. 157) and the other tabulate 

 corals. When mature it forms a simple colony and the 

 zoons are attached by processes which extend from the 

 lower surface. These are seen in PI. 156, figs, i, 2. 



'Sardeson, Ueber die Beziehungen der fossilen Tabulaten zu den 

 Alcyonarien. Neues Jahrb. f. Min., Geol. u. Pal., Beilage Band X, 

 Heft 3, 1896; see also Moseley, Chall. Rep., Zool., II, part 7, 1881, 

 p. 102. 



