PALEONTOLOGY. 97 



the side walls of the cups unite into a common external wall of 

 more massive structure ; only the bottoms of the cups escape the 

 general agglutination of the parts and remain free laminae separated 

 by an interstice from the adjacent cup bottoms. These laminar cup 

 bottoms are, in the description of Cyathophylloid corals, considered 

 under the name of diaphragms. In a portion of the Cyathophylloid 

 family the interstices between the crest-like plications of the end 

 cups are free and open ; in other generic groups the interstices are 

 up to the margins of the calyces traversed and filled with vesic- 

 ulose plates, which divide them into small cell spaces. 



The family of Cyathophylloids is represented by numerous 

 modifications of its type, which will be specially considered in 

 the generic descriptions. Milne-Edwards has adopted thirty-five 

 genera of Cyathophylloids, and a large number of additional gen- 

 eric names, partly synonymous with the above, are used in the works 

 of palaeontologists. A careful study of the various forms has con- 

 vinced me that a large proportion of the promulgated genera have 

 been negligently established upon an examination of insufficient 

 material and with incorrect appreciation of structural characters, 

 individual peculiarities being often mistaken for important generic 

 differences. In passing the genera in review, I will have frequent 

 occasion for rectifications and changes in the arrangement, by which 

 the number of the genera will be greatly reduced. 



Cystiphyllid(B are pointed out as the fourth family of the Zoan- 

 tharia rugosa, composed of the single genus Cystiphyllum. These 

 corals differ from the true Cyathophyllum merely in having but a 

 rudimentary development of the plications, which never compose 

 continuous vertical leaves. The plications of Cystiphyllum are 

 low crests much obscured by the blistered surface of the calycinal 

 walls, which are entirely composed of vesiculose plates. In vertical 

 sections the polyparia appear to be built up by a superimposed 

 succession of layers of vesicles, disposed in accordance with the 

 shape of the end cups. 



An uninterrupted chain of transition forms between Cystiphyllum 

 and the corals of the Cyathophylloid family exists, and the rela- 

 tions of Cyathophyllum proper and Cystiphyllum are so close that 

 I think it unnatural to separate this genus from it as representing 

 a different family type. 



The formations of Michigan inclose a great many corals of the 



