The Relationship of the Tetracoralla to the Hexacoralla. i6i 



TESTS OF THE THEORY OF DIRECT DESCENT FROM 

 TETRACORALLA. 



GEOLOGICAL OCCURRENCE. 



So-called Paleozoic Hexacoralla. 

 Palaeacis and its Relation to the Tabulata. 



A description of the genera Palceacis, Micro cyathus, and Ptycho- 

 chartocyathus, and a Hst of species are given on pages 165-168. 



Before its microstructure became known, Palceacis, which then 

 included the above genera, was supposed to be one of the Porifera 

 or sponges. Its external appearance, the occurrence of small 

 pores in the outer wall, larger pores in the cups, and a system 

 of contorted canals, all rather favor this opinion, but there are 

 other and more important structural conditions that cannot be 

 explained as due to a relationship to Porifera. 



Lack of Spicules. — First, although looked for again and again, 

 no spicules have been definitely seen by any one. This is all 

 the more important as the organisms occur preserved in a variety 

 of ways, although numerous sections have never been possible 

 on account of the scarcity of material. 



Regularity in Budding and Constancy of Shape. — A much 

 more decisive fact and one that removes the genus definitely 

 from the Porifera is that there is a definite law of budding in 

 Palceacis, the same law applying sometimes to more than one 

 species (see PL I, Figs, i and 2). This regularity in budding 

 controls the shape of the individuals so thoroughly that it would 

 be hard to find among corals a more constant shape than that 

 of the type species, P. cuneiformis. There is no such control of 

 individual shape in the Porifera. In no phylum is a greater 

 variety of shapes in the same group possible than in the Porifera 

 and in no phylum is external shape of less systematic importance 

 {cf. Zittel-Eastman 1913:47). The cimeate base which occurs 

 in P. cuneiformis, P. obtusa, P. compressa, and P. cavernosa is 

 a feature seen in some species of Hexacoralla which are not 

 closely attached to their foundation. The hexacoral Endopachys 

 maclurei of the Eocene of Alabama has a cuneate base quite 

 similar to that of P. obtusa. Such a regular exteirnal form as 

 this is again not a character which is to be expected in Porifera. 



Trans. Conn. Acad., Vol. XXI n 1917 



