144 GEOLOGICAL DISTRIBUTION. 



group to which the modern Heliopora, recently claimed to be an 

 alcyonarian, also belongs.* The rugose-corals, apart from the very 

 limited number of forms which occur fossil in deposits newer than 

 the Paleozoic — ?Holocystis (Cretaceous), Conosonilia (Tertiary) — 

 have apparently two living representative types in Guynia and Hap- 

 lophyllia. On the other hand, the modern star-corals, if we exclude 

 from tliis group the Favositidse, have but a feeble development in 

 the earlier deposits, although several recent families are represented 

 (PoritidiE, Eupsammidae, Astrseidae). Of the genera belonging to 

 this group, Protaraea, Stylaraea, Prisciturben, and Calostylis date 

 back to the Silurian period. 



The remarkable development of corals in the Silurian seas makes 

 it not a little difficult to account for the total, or almost total, ab- 

 sence of their remains in deposits of the preceding Cambrian age. 

 It is scarcely credible that the animals of this class should not have 

 already then existed ; but if so, what has become of them ? In ex- 

 planation of this anomaly some geologists have urged that the 

 strata of Cambrian age which contain recognisable fossils are all of 

 a deep-sea origin, and that in the shallow and littoral deposits, 

 where we might be expected to look for the traces of the organisms . 

 in question, organic remains have been completely obliterated 

 through rock-metamorphism of one kind or another. The evi- 

 dence supporting this hypothesis is, however, far from satisfac- 

 tory, and the problem must be considered as one still awaiting 

 solution. 



The graptolites, a group of organisms whose earliest remains 

 are found in the transition rocks which unite the Cambrian and 

 Silurian formations, and whose organisation appears to be most 

 nearly reflected in that of the recent sertularians or sea-firs of the 

 class Hydrozoa, constitute an important element in the Silurian 



* The reference ofFavosites to the Poritidae, it must be confessed, is based 

 upon rather slouder evidence, and perhaps scarcely less so the placing of 

 HelioUtes among the Alcyouaria. Homes ("Elemente der Palseontologie," 

 188i) justly emphasises the artificiality of a classification in which the number 

 and disposition of the septa and tentacles are made the basis for a division 

 into primary groups, and 'in which other equally important characters are com- 

 pletely lost sight of. The relationship existing between past and recent forms, 

 taken in conjunction with the general homogeneousness of oliaracter exhibited 

 by the Tabulata, would seem to imply that the classification of the recent 

 Aotinozoa requires serious emendation. 



