378 GEOLOGY OF TEE BLACK HILLS. 



CEPHALOPODA. 



TETRAB11ANCHIATA 



AMMONITKME. 



Genus AMMONITES Bruguiere. 

 AMMONITES COPDIFORMIS. 



Plate 6, figs. 20-24. 



Ammonites cordiformis M. & H., Proc. Acad. Nat. Sci. Phil., March, 1858, p. 57. 

 Ammonites cordiformis M. & H., Pal. Up. Missouri, p. 122, PI. v, fig. 2. 



Shell attaining a large size, lenticular in a transverse section ; young 

 individuals depressed-bi-convex, or slightly flattened for a short distance 

 on the side bordering the umbilical area, but becoming much more ventri- 

 cose in older individuals from the greater projection of the ventral or um- 

 bilical margin of the volution, the section of the volution in large specimens 

 being nearly an equilateral triangle. Umbilicus of moderate size, the 

 diameter equaling, or nearly equaling one-third the width of the outer volu- 

 tion ; its sides vertical on specimens of a little more than two inches in 

 diameter and larger, but on small individuals it is rounded, the parts of 

 each volution exposed within the umbilicus not increasing in width equal 

 to the increased size of the shell. Volutions sharply keeled on the back, 

 and the edge finely denticulated or transversely ridged. Side of the volu- 

 tions marked by transverse flexuose costa of very variable strength, which 

 have a sigmoidal curvature in passing from the margin of the umbilicus to 

 the dorsal carina, being strongly curved forward for the outer third of the 

 distance, and are increased by bifurcation and intercalation, so as to number 

 from three to six times as many on the dorsal margin as at the border of 

 the umbilicus. On some of the larger specimens the ridges become obso- 

 lete, the surface being only marked by irregular striae, having the same 

 general direction in their passage across the shell as do the costse where 

 they exist. 



The septa are moderately crowded, being placed so as to have the 

 lobes of one pass within the area of that in advance, but not so as to inter- 



