BKYOZOANS OF THE UPPEE HELDERBEEG AISTD 

 HAMILTON GROUPS. 



By James Hall, LL.D. 



[Read, by title, before tlie Albany Institute, March 29, 1881.] 



The present paper is chiefly devoted to the Bryozoans of the Upper 

 Helderberg and Hamilton Groups.* The descriptions in full were 

 communicated in the Thirty-third Report upon the State Museum of 

 Natural History, in January, 1880, but that document has not yet 

 been printed. The present paper is an abstract of the original, with 

 the descriptions abbreviated to conform to the space at my disposal. 



The number of species has been greatly increased by a series of 

 specimens from the Falls of the Ohio river, very kindly communicated 

 to me by Victor W. Lyon, Esq., of Jefferson ville, Indiana; and many 

 of the forms cited from that locality are due to him. In this collec- 

 tion we have not only the expanded celluliferous parts, but the bases 

 or radical portions of the fronds, in a great number of examples ; 

 and these serve not only to aid in the determination, but to confirm 

 the specific distinctions adopted. 



Although unwilling to increase the number of specific designations, 

 the examination of numerous specimens of nearly all the species has 

 left no alternative but to follow the course here adopted. 



We have within a few years discovered the existence of a silico-cal- 

 careous band at the base of the corniferous limestone in western ^ew 

 York and Canada West. This horizon corresponds with that of the 

 Schoharie grit in eastern New York, but the material appears to have 

 been in solution before deposition ; the sea-bed, in this condition, 

 affording a most favorable soil for the growth of Bryozoans ; — the rock 

 now being largely composed of the broken and comminuted fragments, 

 and of larger and more complete portions of these organisms. In the 

 weathered portions of this rock, the original substance of the Bryo- 

 zoan has been dissolved, leaving a sharp, clean impression preserving the 

 most delicate and minute characters of the fossil. 



* I have here included a few forms of Ch^tetes, which, from their sti-ucture, can 

 scarcely be separated from the Favositid^, while the difference between this genus and 

 Trematopora is hardly determinable by any well marlied characters. 



Under the Fenestelltd^ I have not adopted the genus Polypora, thouirh describing 

 species with from two to four ranges of cells on the branches. The numerous examples of 

 this variatioti have rendered the distinction between the genera obsolete ; but the ques- 

 tion will bemorefully discussed in the Report on the State Museum of Natural History. 



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