TRENTON CONGLOMERATE OF RYSEDORPH HILL 39 



CEPHALOPODA 

 SPYROCERAS Hjatt 



Spyroceras bilineatum Hall sp. 



Orthoceras bilineatum Hall. Pal. N. Y. 1847. 1:199, 

 300 



Hall described and figured as Orthoceras bilineatum 

 a shell which Dr Emmons had collected at Rysedorph hill. 

 Whether the specimens have been found in the matrix or in the 

 pebbles, and in which kind of the latter, can not now be 

 •established. The writer has not observed any specimens of this 

 species in the conglomerates of the localities investigated. 



Spyroceras cf. anellus Conrad sp. 



Orthoceras anellus Conrad. Acad. nat. sci. Phil. 

 Proc. 1843. 1:334 



In the gray crystalline limestone thin and very slightly taper- 

 ing fragments of a cephalopod are found, which are character- 

 ized by sharply elevated, equidistant, longitudinal ridges with 

 «mooth interspaces. Forms with a like surface sculpture, but 

 with much less rapidly tapering shells, have been described by 

 Hall from the Beekmantown and Trenton beds as Orthoceras 

 laqueatum. Dr Clarke has shown ^ that the shell of O. 

 bilineatum is not annulated in the apical region^ but is 

 quite strongly marked by alternating longitudinal elevated lines, 

 which as growth advances become finer, while strong annulations 

 gradually develop. These features bring O. bilineatum 

 under the genus Spyroceras of Hyatt. From this fact it 

 seems probable, that Hall's O. laqueatum is only the apical 

 portion of a Spyroceras, probably O. bilineatum. The 

 specimens from Rysedorph hill show only equal longitudinal 

 ridges, but as they represent much younger growth stages than 

 those observed by Clarke — they have a diameter of only 1 to 

 2 mm — it might be inferred that they also belong to Spyro- 

 ceras bilineatum, and that the finer intercalated, longi- 

 tudinal ridges develop only with later growth. This conclusion 



iGeol. Minn. Pal. 1897. v. 3, pt 2, p. 786. 



