i62 W. I. Robinson, 



Calcareous Structure. — A very suggestive coral-like feature 

 is the structure of the calcium carbonate as seen in thin sections 

 of unaltered calcareous specimens. Diverging fibers, arranged 

 in a manner comparable to that in recent corals, show that this 

 material formed the original skeleton of Palceacis. P. cunei- 

 formis, the genotype, does not reveal any definite structure, as 

 it is invariably replaced by silica, but lamellae can be distinguished 

 on the surface, giving the aspect of diverging grooves and ridges. 

 Between the ridges there are pores which open into canals lead- 

 ing into the main cavity. The inner ends of these canals have 

 often been wrongly called mural pores and the genus has been 

 placed on that account with the Tabulata or even removed to 

 the perforate Hexacoralla. 



P. ohtusa occurs with the original structure of the calcium 

 carbonate preserved and with the interstices filled by secondary 

 silica and iron pyrite. Thin sections show the radial lamellae 

 (PI. I, Fig. 3). Microcyathus depressus, a closely related form, 

 shows the same fibrous structure of the calcium carbonate and 

 the inner pores communicating with the contorted canals (PI. 

 I, Fig. 4). * 



Comparison with Tabulata.— The one criterion which has 

 proved satisfactory in distinguishing between Hexacoralla and 

 Tetracoralla is the manner of addition of the secondary septa. 

 This criterion, however, cannot be applied to forms like Palceacis, 

 since they have no definite septa. If the lamellae really represent 

 septa, which is very improbable, their arrangement is obscured 

 by the interlacing of the canals. Enough is known of the cal- 

 careous specimens to show that there are no structural characters 

 which prevent classing these genera with Anthozoa. On the 

 other hand, there are good reasons, stated below, for including 

 them under the sub-class Tabulata in close relationship with 

 Pleurodictyum, Leptopora, and Vaughania. All these genera 

 are characterized by large individuals with thick walls which 

 are traversed by canals very irregular in location and direction. 

 There are no mural pores such as those of Favosites, although 

 apical pores and canal openings give a similar appearance. The 

 manner of budding is by pairs, with one individual slightly in 

 advance of the other. The buds are added laterally in such a 

 way that there may be formed a thin encrusting or globular 

 colony. 



