Polytremacis and tlic Ancestry of Helioporidce. 297 



first part of the skeleton to be formed, and which persists in the adult 

 as the calicular tube or inner tube of the calicle. In the development 

 of a young Heliolites the thecal tube is first formed ; when this tube is 

 complete a series of septa develop from the inner walls of the tube, 

 and then the coenenchyma begins to form on the outer side of it. 



Bourne, on the other hand, gives a very different explanation of 

 the structure of the corallum, and holds that it is fundamentally the 

 same in Heliopora and Heliolites, in both of which the calicle is bounded 

 by a " coenotheca," i.e., a tube formed of the walls of a group of different 

 elements in a colony, secondarily united into a single tube [3, p. 468]. 



Unfortunately nothing is known of the development of the primary 

 calicle in Heliopora, so that no direct comparison of that stage in the 

 two groups is possible. But the comparison of the formation of young 

 calicles on the growing edges of Heliopora affords some suggestive 

 hints. The young calicles in both genera pass through identical stages, 

 which are represented for Heliopoi'a by fig. 6, a-d, and for Heliolites 

 pmvsus by fig. 7, a-g. In both cases the calicular theca of the com- 

 plete calicle represents either the outer walls of the group of caeca 

 which formed the calicle, or was formed by those outer walls being 

 absorbed and re-deposited during the process of coenenchymal gemma- 

 tion. 



A direct comparison of the development of the primary calicles in 

 Heliopora and Heliolites would, no doubt, aff'ord a better basis for an 

 opinion than can be obtained from the development of young calicles 

 in old coralla. But until zoologists work out the development of 

 Heliopora, we can only appeal to the comparison of young equivalent 

 calicles, and they develop on the same lines. 



Hence, though nervous at differing from two such authorities as 

 Professor Lindstrom and Dr. Hinde, I am bound to confess myself 

 unconvinced that any essential difference between the Helioporidae 

 and Heliolitidae has yet been established. Accordingly it is not 

 unreasonable to expect in Mesozoic deposits some connecting links 

 between the living and Palaeozoic representatives of the group. 

 Polytremacis appears to me to be such an intermediate form. F. septifera, 

 with its eight to fourteen or twenty well-developed septa, agrees with 

 Heliolites, differing by the less regularity in the number of septa. The 

 Turonian P. hlainvillei and the Eocene P. bellardi agree with Heliopora, 

 as the septa are reduced to septal ridges. 



If we place any species of Polytremacis in the Heliolitidse,"^ that 

 family can no longer be described as characterised by the possession of 

 twelve septa. If, on the other hand, we place P. septifera in the 

 Helioporidse, we have to admit in that family the presence of septa 

 as well-defined as they are in some Heliolitidae. In either case the 

 distinction between the two families, based on the septal characters, 

 * As suggested by Jfeumayr, 11, p. 321. 



