FOSSILS OF THE NIAGARA GROUP. 411 



the ventral side. Outer chamber rapidly contracting from near the last 

 septum to the aperture, giving it a somewhat conical form, the length 

 being less than the transverse diameter of the septum. Aperture 

 trilobed, the ventral opening small ; dorsal opening moderately large and 

 subtriangular ; the proportions of the transverse and axial diameters are 

 as three to four. Septa moderately deep, with a very regular convexity. 

 Siphuncle situated at one-third the distance from the ventral margin to 

 the centre. Surface marked only by irregular lines of growth. 



Formation and Locality. — In limestone of the Niagara group, at Bridge- 

 port, Illinois. 



GENUS ORTHOCERAS, Bretn. 

 Obthoceras annulatum, Sowerby. 



PLATE XX, FIGS. 4, 5, 6 ; PLATE XXIV, FIGS. 2, 3, 4. 



Orthoceras annulatum, Soweebt. Mineral Conchology, II, p. 77, Tab. cxxxiii. 1818. 

 Orthoceratites undulatus, Hisingee. Anteckn. V, Tab. iv, fig. 6. Vet. Akad. Handlingar, Tab. 



vii, fig. 8. 1826. 

 Orthoceratites undulatus, Hisingee. Lethea Suecioa, p. 28, Tab. x, fig. 2. 1827. 

 Orthoceras annulatum, Muechison. Silurian system and Siluria. 

 Orthoceras undulatum. Hall. Pal. New York, II, p. 293, Plates Ixiv, Ixv. 

 Orthoceras nodocostum, M'Chesnet. New Palaeozoic Fossils, p. 94, Plate 9, fig. 5. 1861. 

 Orthoceras nodicostatztm, M'Chesnet. Chicago Academy of Sciences, Vol. i, p. 53, Plate ix, fig. 5. 



This species is of common occurrence in the shales of the Niagara 

 group in New York. It is found in the lower beds exposed at Waukesha 

 and near Wauwatosa, in Wisconsin. The specimens are casts, usually 

 preserving only the marks of the strong annulations, which vary con- 

 siderably in their sharpness and degree of elevation ; but there is no 

 evidence of specific distinction among the forms of this character. The 

 fine undulating transverse striae are preserved in the impressions of the 

 exterior surface, associated with the casts. 



Orthoceras column are. Hall. 



PLATE XIX, FIGS. 4-6, 8. 



Orthoceras columnare. Hall. Rep. Progress Geol. Survey Wisconsin for 1859. Feb., 1860. 

 Compare O. canaliculatum, Sowerby, Silurian System, T. 13, fig. 26. 



The species described by me from Wisconsin is elongate cylindrical, 

 very gradually tapering. The siphuncle is central, of medium size, and 

 not expanded between the septa. The septa are distant about one half 

 the diameter. 



