Tabulate Corals with Existing Species. 359 



But among the Tabulate corals, after excluding the Mille- 

 2)oridce, great diversities of structure still remain ; and no doubt 

 representatives of several families that ought to be widely se- 

 parated in a natural system are thus combined together on 

 account of a single unimportant character. Many of these 

 genera are extinct and apparently have no very closely allied 

 representatives among living corals. The affinities of such 

 genera may long remain doubtful. But in other cases there 

 are living corals having very close relations with certain 

 Palaeozoic genera, and these we are even now able to classify 

 with as much certainty as we can the ordinary forms of existing 

 corals. 



Among the best-known of the tabulate corals are the nume- 

 rous species of Pocillojjora and allied genera, which evidently 

 constitute a distinct family {Pocillojwridte), largely represented 

 in the tropical waters of the Pacific and Indian oceans. 

 These corals are characterized by rather small tubular cells, 

 usually with 6, 12, or 24 radiating septa, w^hich, even in the 

 same specimen, may be obsolete in some of the cells, by im- 

 2)erforate compact walls, and by a more ' or less abundant 

 compact coeneuchyma between the lateral cells, which may, 

 however, be absent Avhere the cells are crowded, as at the ends 

 of the branches. The writer has shown in several previous 

 papers* that the Pocillojjoridce are the corals of true polyps. 



type of Rugosa, and not to the family of Fungians, it 'becomes evident 

 that in their order of succession from the Mesozoic era, in which they 

 first make their appearance, the gi-eat types of the class of Polyps have 

 succeeded one another in the following order : — first Turbinoliaus, next 

 Fungians, next Astrseans, and last Madrepores — in exactly the sequence 

 in which these types stand to one another, as far as their structural gra- 

 dation is concerned, and in exactly the same order in which, dui'ing their 

 growth, these corals pass from one stage to another." 



But on the other hand, since we now find that the Acalephian affinities 

 of the Rugosa and most of the Tabulata are wholly imaginary and without 

 the slightest foundation in nature, all this beautiful theory of geological 

 sequence falls to the ground, and we find the Madrepores represented 

 even in the Lower Silurian rocks by Favosltes and other Alveopora-\\]ie 

 forms, which are certainly neither luio nor emhri/onic types ! 2\.ud even 

 in JMesozoic times the Astrteans appear in force quite as early as the 

 Fungians or the Turbinohans. If there has ever been such a definite 

 geological sequence of the groups of corals as Agassiz imagines, it must 

 have taken place in the ante-Silurian ages, concerning the life of which 

 we know nothing. In the Lower Silurian seas the order was already well 

 developed and highly diversified. 



* " On the Affinities of the Tabulate Corals," in Proceedings of the 

 American Association for Advancement of Science, 1807, p. 148. Pro- 

 ceedings of the Essex Institute, vol. vi. p. 90, 1869. Transactions of the 

 Connecticut Academv, vol. i. p. 518, 1870. American Journal, vol. i. 

 p. 389, May 1871. 



