GENUS ORTHOCERAS. 353 



ORTHOCERAS MEDULLARE. 



PLATE 20 (11), FIGS. 1 & 2. 



Orthoceras medullare : Hall, Report of Progress Geol. Survey Wisconsin, 1859 

 (Pub. Feb. 1860). 



Shell cylindrical, often a little compressed, gradually, and in some speci- 

 mens more rapidly tapering. The septa are distant nearly half 

 the diameter, but are subject to considerable variation in the same indi- 

 vidual, so that nearly three chambers are sometimes included in a length 

 equal to the diameter. The siphuncle is large and slightly expanded 

 between the septa. 



The surface is marked by strong sharp subequal longitudinal striae, 

 which are cancellated by fine transverse striae. The longitudinal stria3 are 

 often alternated by finer sharp striae in the same direction. Surface of the 

 cast smooth, and by this character it is distinguished from the casts of 

 O. columnare and O. angulatum. 



Geological formation and locality. — In limestone of the Niagara group 

 at Waukesha and Wauwatosa, Wisconsin. 



ORTHOCERAS ANGULATUM. 

 PLATE 19 (10), FIGS. 10 & 11. 



Orthoceras angulatum : Wahlenberg, Nova Acta, p. 90. 



" *' HisiNGER, Lethea Suecica, T. X, fig. 1. 



" virgatum : Sowerby, SiL System, T. IX, fig. 4. 



" virgatum? Pal. N. Y., VoL ii. page 291, PL Ixiii, fig. 2 and 3. 



Compare 0. canaliculatum ; Sowerby, Sil. System, T. 13, fig. 26. 



Specimens from Wisconsin are apparently identical with those of New- 

 York referred as above ; the former being casts of the interior, while the 

 latter are preserved in a soft calcareous shale, and have the surface mark- 

 ings more or less obscured. 



The septa are distant about one fourth the diameter of the shell, the 

 siphuncle is central or subcentral, with scarcely an apparent expansion 

 between the septa. 



The longitudinal ridges are angular, and about one line distant when the 

 shell is an inch in diameter. The finer surface striae are but imperfectly 

 preserved on the cast, and it is only in the impressions of the exterior that 

 these markings become conspicuous. 



This is probably the species described by Mr. M'Chesney, in a paper 

 published in 1861, under the names O. scammoni, O. hoyi, O, lineolatum^ 

 O. irregulare, O. woodworthi. The last one figured in a fragment less 

 than an inch in length. A gutta percha cast sent by Prof. Winchell 

 under the name O. Hcammoni, corresponds very well with specimens referred 

 by me to O. angulatum. Should the species prove distinct from the 

 European one, we may select a name from among those above cited. The 

 comparison of a considerable collection of specimens from Bridgeport and 



[Assem. No. 239. j 45 



