NIAGARA LIMESTONE OF CHICAGO, ILLINOIS. 101 



size ; and no mention is made of the very conspicuous dorsal nodes. McChesney has also 

 described B. chicagoensis from the same quarries, but he expressly states that the cast is 

 destitute of a carina. Our species may also be compared with B. (Bucania) angustata 

 Hall, as figured in the Geology of Canada, p. 344. There is no comparison, however, be- 

 tween the apertures or dorsal characters of the two. 



Orthoceras Scammoni McChesney. New Paleozoic Fossils, p. 92. Specimens unusually 

 well preserved. We may add to the characters given by McChesney, the following : The 

 siphon is .52 inch in diameter, at a place where the shell has a diameter of 1.77 inch. In 

 the same specimen, at the distance of four inches, the siphon is .5 inch in diameter, and the 

 shell has become 1.38 inch. The siphon tapers less rapidly than the shell, and is about one 

 third the diameter of the shell. In the same specimen the diameter of the larger end is 

 2.2 inches, being the largest specimen seen. Nine and a half inches from here the diameter 

 has diminished to one inch. The distances of four successive septa, where the mean diam- 

 eter of the shell is 1.4 inch, are .34, .45, .41, .40 the mean being .40. Strife of growth 

 two and a half to three in the space of one tenth of an inch. 



This species closely represents 0. cancellatum Hall, but we have not been able to detect 

 any longitudinal striae between the ridges. 



Orthoceras nodocostum McChesney. New Pal. Fossils, p. 94. Our specimens enable us to 

 complete McChesney's description. 



Shell very gradually tapering ; annulated by a raised rounded ring corresponding to each 

 chamber, and situated a little in front of the middle of the chamber. Intervals between 

 the rings regularly concave. Septa rather deeply concave, crossing the shell with a de- 

 viation of three or four degrees from a right angle ; their distance asunder being one 

 third the diameter of the shell. No annular strias can be seen in our specimens. The 

 longitudinal ridges are barely discernible on the shell, and leave no trace upon the cast. 

 The siphon is cylindrical, and about one fourth the diameter of the shell. 



The distances apart of four successive septa, in a region where the mean diameter is .65 

 inch, are .20, .22, .22, .21, giving a mean distance of .21 inch. 



The absence of concentric striae, and the presence of faint longitudinal ridges, distinguish 

 this species from 0. undulatum Hisinger. 



Orthoceras Laphami McChesney, New Pal. Foss. p. 91. 



GOMPHOCERAS Sowerby. 



Gomphoceras Marcyse W. and M. 



Plate III. figure 8. 



Shell of moderate size, perfectly straight. The outer chamber, in connection with the 

 penultimate chamber, gives an outline which is neatly and symmetrically elliptic, with a little 

 flattening at each end. That portion of the shell between the penultimate chamber 

 and the apex is gradually tapering. Transverse section elliptic, with transverse axis to 

 dorso-ventral as 27 to 29 or 30. Apertural extremity constricted in a regularly increasing 

 ratio to the ventral lobe of the aperture, beyond which the constriction proceeds on the 

 sides, in the same manner as far as the lateral lobes of the aperture. The aperture is trisin- 

 uate ; the two lateral sinuses are situated a little back of the central axis, and are separated 

 from each other by a transverse oral axis, equal to one half the greatest transverse diam- 



6 



