838 CRETACEOUS PALEONTOLOGY. 



1905. Ammonites vanuxemi Johns., Proc. Acad. Nat. Sci. Phil., 

 1905, p. 27. 



Description. 'Shell attaining a rather large size when full 

 grown, a large individual in the collection of the Philadelphia 

 Academy of Science having a maximum diameter of over 180 

 mm.; moderately compressed, subdiscoid in form, with five or 

 six volutions; outer volutions embracing the inner ones a little 

 less than one-half their width; the umbilicus rather broad, the 

 volutions with a distinct umbilical shoulder; the venter flattened 

 with a distinct keel which becomes less prominent with age ; the 

 margins of the flattened venter each marked by a row of obliquely 

 elongate nodes which are formed by the extremities of the 

 rounded costae which cross the sides of the volutions; a row of 

 nodes also marks the inner extremities of the costae along the 

 margin of the umbilicus, and three other rows occur at nearly 

 equal distances apart between the umbilical and marginal rows; 

 all these markings are more strongly developed upon the younger 

 individuals, becoming more and more faint with increasing age; 

 sides of the volutions in younger individuals nearly flat, or only 

 slightly convex, becoming more strongly convex in older indi- 

 viduals. Septa composed of a large ventral lobe divided medially 

 by a low, truncated siphonal saddle, and three lateral lobes, the 

 middle one of which is much the larger with about five divisions 

 and many serrations; the first lateral lobe is narrow with ser- 

 rated sides, its depth about one-half that of the ventral lobe ; it is 

 really little more than a rather deep and narrow indentation of 

 a broad first lateral saddle; third lateral lobe larger than the 

 first but much smaller than the second; on the dorsal side, at 

 the bottom of the dorsal furrow, is a deep, narrow, serrated 

 dorsal lobe, with' two smaller lobes on either side of it between 

 the dorsal line and the umbilical shoulders; the divisions of the 

 saddles much more rounded than those of the lobes. 



Remarks. Whitfield has recognized Morton's two species, 

 A. delawarensis and A. vanuxemi, as distinct, the chief difference 

 being in the greater compression of A. vanuxemi and its smaller 

 size. The two forms occur together in the Merchantville clay- 



