INVERTEBRATE PALAEONTOLOGY. 



473 



Subgenus SPHENODISCU.v, Meek. 

 Place ii ticeras lenticulare, Owen (sp.). 

 Plate 34,iigB. 1,(1,6, c. 



Ammonites lenticularis, Owen ( 1852), Report I*. S. Geo]. Survey Iowa, Wisconsin, anil Minnesota, r.T'.t, tab. 



viii, fig. 5 (not .1. lenticularis, Phillips, 1S25, nor von Bncb). 

 Ammonites lobatus, Tnomey (1854), Proceed. Acad. Nat. Sci. Philad., VII, 166.— Meek and Hayden (1856), 



ib., VIII, 280.— Gabb (1861), Synop. Moll. Cret. Group, 13.— Meek (1864), Smithsonian 



Check-List N. Am. Cret. Fossils, 24. 

 Ammonites Purdenalis, Binkhorst (1861), Mouogr. des Gast<?ropodes et des Cephalopodes de laCraie super. 



dn Limbourg, 21 (not von Buch (1849), Ueber Ceratiten, 31, tab. vi, figs. 8,9,10). 



Fig. 66. 



Placenticeras (Sphenodiscus) lenticulare. 

 A small specimen, photographed on wood, and cut natural size, to show the appearance and 



crowding of the septa in shells of this size. 



Shell attaining a large size, compressed-lenticular ; umbilicus very small, 

 or nearly closed ; volutions very broad in their dorso-ventral diameter, which 

 is twice and a half that of their greatest convexity near the middle, from near 

 which their sides converge with very slight convexity, to the more or less 

 acutely cuneate periphery ; each of the inner turns entirely embraced and 

 hidden within the profound sinus on the umbilical side of the next succeed- 

 ing outer one; aperture narrow-sagittate; lip unknown; surface smooth, or 

 with about fourteen very obscure, radiating, slight prominences or ridges, 

 around the outer half of each side of each turn ; ridges very rarely each show- 

 ing a slight tendency to develop an obscure tubercle at its inner end. 



Greatest diameter of the largest specimen seen (consisting entirely of 

 fin n 



