CEPHALOPODA. 159 



ured on the inside curvature. Transverse section of the shell obtusely subtriangular, 

 flattened or but slightly convex on the inner surface, rounded on the lateral surfaces, 

 and obtusely rounded on the back; the dorso-ventral and lateral diameters are about as 

 four and five, and the triangular form is more perceptible in the earlier stages of growth, 

 owing to the greater convexity of the inner face in the upper portion and on the outer 

 chamber. Outer chamber comparatively short, being about two-thirds as high as wide. 

 Aperture large, irregularly tri-lobed, straight on the inner face and about four-fifths 

 as wide as the entire width of the shell, and apparently about two-thirds as wide in a 

 dorso-ventral direction as laterally. The exact form of the aperture on the outer side 

 cannot be ascertained, owing to the imperfection of the specimen in this part. Septa 

 moderately concave, very closely arranged in the lower part but more distinctly dis- 

 posed above; the rate of increase in distance somewhat gradual to near the upper portion, 

 where two or three 'of the septa are slightly more crowded. In the more 

 distant portions three chambers occupy the space of one inch, but in the 

 lower part of the specimen, where the transverse d'ameter is a little more than one and 

 a half inches, they are less than one-twelfth of an inch apart. Siphuncle of moderate 

 size and subcentrally situated. Surface of the shell unknown." 



Remarks — A specimen from the Grand Tower limestone probably belongs to this 

 species though the lack of diagnostic characters of the group makes positive identifica- 

 tion impossible in most cases. The specimen is larger than the one figured by Whitfield 

 and it does not show as much curvature as that specimen. More of the smaller end is 

 missing than in the Ohio specimen. The aperture, shape, size, and living chamber are 

 much alike in the specimens from Ohio and Missouri. The depth of the smaller chambers 

 is not as small on the Missouri specimen, but the transverse diameter at the small end is 

 13^2 inches in the Ohio specimen and more than two inches in the Missouri specimen. 



Occurrence — Grand Tower limestone, 3 miles south of St. Marys. 



Fig. 10. Outline of the aperture of Gomphoceras hyalti Whitfield. 



Gomphoceras cf. impar Hall 



Plate 37, figure 1. 



1879. Gomphoceras impar Hall, Pal. New York, vol. V, pt. 2, p. 332. w 

 1888. Gomphoceras impar Hall, Pal. New York, supplement to vol. V, pt. 2, in vol. VII, 

 pi. 120, fig. 4; pi. 121a, fig. 1. 



Hall's description — "Shell straight, exogastric. Transverse section broadly oval. 

 Point of greatest transverse section near the base of the chamber of habitation. Tube 

 regularly enlarging to the last septum, where it is slightly constricted. Apical angle 

 about 30°. 



Chamber of habitation large, with a length equal to the greatest ventro-dorsal di- 

 ameter. The dorsal side slopes somewhat more abruptly toward the aperture than the 



