246 NEW YORK STATE MUSEUM 



these structures is immature in the sense that they characterize this 

 primary manifestation of species of Rensselaeria. This is their 

 expression, for example, among the species of the Helderbergian 

 fauna. On the other hand the Rensselaerias from Presque Isle 

 stream are in these respects up to the full development of the type 

 of the genus, R. o v o i d e s. These characters in such condition 

 do not therefore indicate a primitive phase nor an early stage in 

 the history of the genus. 



The shells from Edmunds Hill are of more primitive expression, 

 especially in hinge structure, the plate not being thickened though 

 well developed and separated medially or perforated, in this respect 

 having the structure of the early species of the genus, such as 

 occur in abundance in the beds of the Helderbergian of New 

 York. This shell is in a general way smaller and carries within 

 itself the expression of retarded development with reference to the 

 larger forms at Presque Isle. I will not venture the statement 

 that the small forms do not occur at Presque Isle but the larger 

 have not been observed at Edmunds Hill. 



The similarity of these smaller forms with the R. stewarti 

 of Dalhousie is very close yet it seems to me improper to unite 

 the shells, for such union would lead to the identification of the 

 still simpler Dalhousie shell with the progressed form from Presque 

 Isle. At Dalhousie the species seems to have become fixed in 

 its primitive details ; conditions in the Chapman Plantation region 

 have permitted progress beyond the expression of R. stewarti. 



The especial expression of the generic type of Rensselaeria 

 afforded by these two closely allied species is repeated in the shell 

 R. portlandica Billings from the Square Lake limestone of 

 Maine. The last opportunity which the writer had for critical 

 examination of the type of this species was while studying an 

 extensive series of Rensselaeria and brachiopods allied thereto,, 

 in the preparation of Paleontology of Nezu York, volume 8, part 2. 

 It was then observed that the species Terebratula gaudryi 

 d'Orbigny, the type of Bayle's genus Trigeria, was probably present 

 in the Oriskany fauna of Maryland. This is a strongly plicated 

 rensselaeroid, throughout of similar aspect to these under considera- 

 tion save in minor details. To the same group R. portlandica 

 belongs and in the work cited was referred to the genus Trigeria. 



The genus Trigeria means a strongly plicated rensselaeroid with 

 the hinge plate in an elemental condition, i. e. perforated, but with 

 cardinal process slightly developed if present at all, and a cardinal 

 area more or less distinctly retained on both valves. The genus 



