PALEONTOLOGY. 429 



on the opposite side, in the more perfect specimen, but may not be constantly so in 

 all individuals. Septa shallow, arranged at nearly equal distances from each oiher 

 in the larger parts, and numbering about seven in an inch, except near the outer 

 chamber, where there are usually one or two more closely arranged. The outer 

 chamber is proportionally short, and rapidly contracted in the upper part to about 

 one-half the diameter below, to form the transversely sub-triangular or obscurely 

 trilobed aperture, which is rounded at the lateral extremities, straightened on the 

 dorsal side, and provided with a moderately deep but rather narrow sinus on the ven- 

 tral margin. Siphuncle proportionally small, and situated close to the dorsal side. 



Only two individuals have thus far been observed, and these show 

 some slight variation in the form of transverse section and in the propor- 

 tional length of the outer chamber; the one retaining the chambers 

 being shorter above, and more flattened on the dorsal side than the other. 

 In this specimen, the septa are somewhat obliquely arranged, being high- 

 est on the dorsal side, which may, however, be owing to oblique com- 

 pression in the matrix. The individuals, being both internal casts, have 

 afforded no opportunity of observing the surface structure. 



Formation and Locality. — In the limestone of the Upper Helderberg 

 group, at Smith and Price's quarries, near Columbus, Ohio. Collected 

 by Mr. Hyatt. 



Genus CYRTOCERAS Goldf. 



Cyrtoceras cretaceum. 



Plate IV, figs. 2 and 3. 

 Cyrtoceras cretaceum Whitf., Ann. N. Y.. Acad. Sci., March, 1882, p. 209. 



Shell of medium sire, somewhat moderately expanding in its upward growth to 

 the base of the outer chamber, from which point it again contracts to the aperture; 

 the increase not always regular, but in some individuals more abruptly expanding 

 above than below. Shell slightly curving throughout its length, appearing less 

 arcuate in the upper portion, owing to the contraction of the outer chamber toward 

 the aperture. Transverse section oval, widest in a lateral direction, and with the 

 inner surface much less arcuate than the outer or dorsal surface. Outer chamber 

 proportionally short, the length not exceeding the dorso-ventral diameter of the 

 lower end ; margin simple, so far as can be determined from any of the specimens, 

 showing only a broad, shallow sinuosity on each side. Septa somewhat closely 

 arranged and deeply concave, but slightly increasing in distance in the upper part, 

 the average length of the chambers being about one-tenth of an inch, but some- 

 what more crowded just below the outer one. Siphuncle of moderate size, situated 

 a little within the dorsal surface, and very slightly expanded within the chambers. 

 Surface of the shell marked only by transverse lines of growth parallel to the mar- 

 gin of the aperture. 



The shells are moderately abundant, and show slight variations in form 

 among individuals, especially in the rate of increase in dimensions or in 

 the regularity of the expansion, as well as in the comparative distance 

 between the septa; a single individual showing a much greater distance 

 between them in the upper part of its length. The shell would probably 

 be considered by some as belonging to the genus Oncoceras, as the de- 

 crease in diameter in the upper part of the outer chamber gives to the 



