CLASSIFICA TION. 2 5 



tions, and that, owing to the great density of its tissues, even 

 the finest of these leave certain important points obscure. I 

 shall have to deal with the orenus at greater length at a later 

 period, and will only say here that it seems to form in some 

 respects a link between the Perforate Corals and the Alcyo- 

 narian family of the HelioporidcE. It resembles the former in 

 the fact that the visceral chambers of contienous corallites are 

 placed in communication by a well-developed system of hori- 

 zontal canals ; and, on the other hand, it approaches the latter 

 in the fact that the ordinary polypes are surrounded by what has 

 been generally regarded as a tubular " coenenchyma," though 

 not truly of this nature. The so-called " ccenenchymal tubuli " 

 — judging from the analogy of the recent Hcliopora — are really 

 tenanted by special zooids, and are therefore not truly ccenen- 

 chymal. In the possession, then, of a series of large polypes 

 surrounded by a much more numerous series of smaller special- 

 ised polypes, Thccia agrees with Heliopora and Heliolites; and 

 the genus is therefore probably Alcyonarian. It differs, how- 

 ever, from the above in the fact that the corallites do not possess 

 distinct walls ; the septa of the large tubes are broad and tooth- 

 like, and the interstitial tubuli are apparently destitute of 

 tabulae, irregular, and opening on the surface by very minute, 

 often stellate, apertures ; while the larger polypes are directly 

 connected by lateral canals. These peculiarities fully entitle it 

 to be regarded as the type of a special family. 



X. Helioporid/E. — This family has been founded by Mr 

 Moseley (Phil. Trans., vol. clxvi. p. 92, 1876) for the recep- 

 tion of the extraordinary recent Hcliopora, and the long ex- 

 tinct Heliolites, Plasmopora, and allied forms. The corallum 

 in this family (fig. 11) is composed of two sets of corallites, a 

 larger and smaller — the latter hitherto, but wrongly, termed 

 " ccenenchymal tubuli." The larger tubes are furnished with 

 delicate septa (generally twelve in number), and are traversed 

 by remote complete tabulae (fig. 11, c). The smaller tubes 

 everywhere surround the larger ones, have no septa, and are 

 crossed by more numerous tabulae. Heliolites and most of its 



