3!)0 Systematic Paleontology 



shells from their matrices without injuring them. The form has been con- 

 fused in the synonymies with S. lenticularis Owen, from the Fox Hills 

 Group of the Upper Missouri. The east coast form differs from that of 

 the interior by the greater complexity of the sutures, particularly the 



saddles. 



Occurrence. — Monmouth Formation. Brightseat, Brooks estate near 

 Seat Pleasant, Friendly, 1 mile west of Friendly, McISTeys Corners, Prince 

 George's County. 



Collection. — Maryland Geological Survey. 



Outside Distribution. — Magothy Formation. Cliffwood clay, New 

 Jersey. Matawan Formation. Merchantville clay marl, Woodbury clay, 

 Marshalltown clay marl. Wenonah sand. New Jersey. Peedee Sand. 

 North and South Carolina. Ripley Formation. Exogyra costata zone. 

 Extreme top of zone, Lowndes, Tippah and Union counties, Mississippi. 

 Selma Chalk. Exogyra costata zone, Starkville, Mississippi. Fox Hilts. 

 Western Interior, Aff. Sphenodiscus lenticularis. 



Family PR10NOTROPIDAE 



Genus MORTOMCtRAS Meek 

 [Inv. Pal., vol. ix, 1876, p. 448] 

 Type. — Ammonites vespertinus Morton. (= A. lexanus Roemer.) 

 " Shell discoid ; periphery with a simple, low, central keel, and a more 

 or less defined sulcus on each side of it, the sulci being generally each mar- 

 gined externally by a row of compressed nodes ; umbilicus wide ; volu- 

 tions narrow, slightly embracing, and ornamented by regular, simple, 

 straight, tuberculated costa\ Septa in the typical species with three 

 lateral lobes on each side, the first one being longer than the siphonal lobe, 

 with tripartite extremity, the terminal division being deeply bifid; the 

 second and third lobes much smaller and more or less tripartite or dentate ; 

 first and second lateral sinuses more or less nearly equally bipartite or 

 bilobate at the ends. Shells of this genus will be distinguished from the 

 restricted genus Ammonites by their always single peripheral keel, 



Etymology: "Dedicated to Dr. Samuel George Morton, deceased." 



