TRENTON CONGLOMERATE OF RYSEDORPH HILL 37 



sion, occupyingabout half the width. Lateral angles subacutely 

 angular. Aperture unknown, but judging from the closely 

 crowded growth lines, which curve forward on the dorsal 

 side more than on the ventral, the peristonie can hardl}* have 

 arched forward on the ventral side, but probably was more 

 or less abruptly truncated (subgenus Orthotheca N'ovak) Oper- 

 culum unknown. 



Dimensions. Length 35 mm or moire, width 10+ mm, hight 

 6A~ mm. 



Observations. Only one other species has been made known 

 from the Trenton, Hyolithus baconi Whitfield^. The 

 iJysedorph hill species differs from this in tapering less rapidly, 

 having a more elevated dorsal side (according to the 

 lateral view given of H. baconi), and the growth lines on 

 the convex side arch forw^ard instead of passing straight across. 

 It is noteworthy that only two species of this genus have thus 

 far been found in the rich* Trenton faunas from various regions 

 of the continent. The distribution of this genus in the Ameri- 

 can beds seems to furnish an instance of the intermittent 

 appearance of a group of organisms; for Hall- cites no less than 

 nine species from the Cambric and six from the Devonic beds, 

 with only one species to fill the tremendous interval from the 

 Potsdam to middle Devonic beds. Since Hall's publication the 

 number of North American Cambric species has increased to 14, 

 ^nd that of the Devonic to 11, while one species has become 

 known from the Chazy, one from the Trenton, one from the 

 Lorraine, and one from the Niagara beds. Nor can it be held 

 that one of the two subgenera into which the genus has been 

 <iivided by Holm comprises the Cambric and the other the Devo- 

 nic species, thus resolving the apparent twofold culmination of 

 the genus into the culmination of two different successive 

 groups of fossils. That the genus Hyolithus is also represented 

 in other localities within this state becomes apparent from Dr 

 White's investigations of tne Trenton Falls section, whence he 



1 Geol. Wis. 1S82. 4:225. 



2 Pal. N. Y. 18T9. v. 5, pt 2, p. 197. 



