1 86 IV. I. Robinson, 



reaching a maximum in the Middle Silurian and Devonian. They 

 continued as a vigorous line in the Mississippian, as the great 

 coral reefs in England and western Europe testify, for the 

 variation of forms in these localities is almost as great as that 

 of the Devonian reefs in North America such as those of Alpena 

 and Petoskey, Michigan, and the Falls of the Ohio at Louisville, 

 Kentucky. In Permo-Carboniferous time a great restriction 

 took place and the number of individuals as well as of genera 

 was greatly reduced. With the end of the Permian record the 

 history of the Tetracoralla closes, although some peculiar Triassic 

 and Jurassic forms have been found to possess a few of their 

 characteristics. No case has yet been recorded, however, of a 

 Mesozoic or recent coral in which the tetracoral septal addition 

 follows the laws of Kunth and Faurot. 



Evolutionary Trends in Tetracoralla. 



Lower Paleozoic. — The different families of the Tetracoralla 

 were in the main established by the Middle Silurian. The 

 Palaeocyclidse and Cystiphyllidae seem to have been aberrant 

 lines which transgressed the limits of favorable variation and 

 so were destroyed. The Palaeocyclidae occurred in the Silurian 

 and Devonian; the Cystiphyllidae began in the Silurian and 

 continued into the Carboniferous. The Cyathaxonidae were the 

 most conservative of all the families of Paleozoic corals, fol- 

 lowing the same pattern from the Ordovician genus Petraia to 

 Polycoelia of the Permian (Zechstein), and also show an evi- 

 dence of a lack of progressive variation, or, possibly even a 

 retrogradation, in the Permian, in that Polycoelia corresponds 

 more closely to the original pattern of Petraia than do the 

 Devonian members of the family. This is therefore obviously 

 not a family from which a new stock should be expected to 

 arise in the early Mesozoic. The two remaining families, the 

 Zaphrentidse and the Cyathophyllidae, were the only ones which 

 approached the end of the Paleozoic with the probability of 

 survival. Both of these vigorous lines began in the Ordovician ; 

 gained a maximum of development in the Silurian which was 

 sustained through the Middle Devonian, and both continued to 

 produce important variations during the growth of the Lower 

 Carboniferous coral reefs. 



