r»8 NEW YORK STATE MTSEL'M 



Observations. 'J'liis funn is very similar tu some other Ameri- 

 can species, viz Uemopleurides canadensis Billings, 

 from the Canadian Chazy beds, and to the only bther Trenton 

 congener, R. s t r i a t u 1 u s Walcott, from the upper third of the 

 Trenton limestone. With both it has in common the tuber- 

 culated cranidium, the presence of three jiaire of smooth linear 

 glabellar furrows and the central tul)ercle on the occipital ring. 

 From both of these it differs in having the cranidium considerably 

 broader than long, a longer tongue-shajied frontal lobe, in possess- 

 ing tine striae, besides the tubercles on the cranidium, and by the 

 strong tuberculation of the posterior part of the occipital ring. 



From R. s t r i a t u 1 u s it also differs in the structure of the 

 pleurae, which are here provided with distinct fulcral tubercles 

 and corresponding notches and an oblique furrow, while those of 

 R. striatulus are described as simply flat. In these fea- 

 tures it agrees strongly with the species described from the 

 Cai-adoc beds of Ireland by Portlock (emended by Salter) as R. 

 colbii, laterispinifer and dorsospinifer. 



In several features it seems to ju<?tify its intermediate strati- 

 graphic position between the Chazy and the upper Trenton forms. 

 Thus it resembles the former (R. ca n a d e n s i s) by the great 

 relative width of its cranidium. and the latter by the character of 

 the glabellar furrows. But the species from Rysedorph hill is 

 still more similar to an Irish form, R. tuberculatus, lately 

 described by F. R. C. Reed^ from the Tramore limestone of county 

 Waterford. With this it has the great width of the cranidium. 

 the shape and extension of the abruptly projecting frontal tongue, 

 the sudden increase in width of the piilpebi-al lobee and the tuber- 

 culation of cranidium and neck segment in common. The Irish 

 form differs in size, being about twice as large asR. linguatus 

 and in the uniformity of its tubercles. It is also said to have 

 but two pairs of glabellar furrows; as, however, some specimens 

 of the R\-sedorph hill species show but two or even but one pair 

 of glabellar furrows, it is quite probable that more complete 

 material of the Irish species would also show the third pair of 



* (^uar. Jour. g(!Ol aoc MM. 56:748. 



