Maryland Geological Survey 385 



" At least in external characters, S. pulcherrimus of Roemer, cited 

 doubtfully in the foregoing synonymy, seems to be very closely allied to 

 this species, and I am much inclined to believe that a comparison of speci- 

 mens may show them to be identical. As Roemer does not figure the septa 

 of his shell, however, and only illustrates the exterior of an imperfect 

 specimen, this question can only be settled by a direct comparison of the 

 shells themselves, or of good figures." — Meek, 1876. 



Occurrence. — Monmouth Formation. Brightseat, Prince George's 

 County. 



Collections. — Maryland Geological Survey, Philadelphia Academy of 

 Natural Sciences. 



Outside Distribution. — Ripley Formation. Exogyra costata zone, 

 Prairie Bluff, Alabama. Fox Hills, Western Interior. 



Genus PLACENTICERAS Meek 

 [Proc. Amer. Phil. Soc, vol. xi, 1870, p. 429] 



Type. — Ammonites placenta DeKay. 



" Shell with the very narrow periphery truncated, and often provided 

 with a row of compressed alternating nodes along each margin; volu- 

 tions each about three-fourths embraced by the next succeeding outer 

 one ; septa with the lateral sinuses provided with more or less branched 

 and digitate terminal divisions, umbilicus small or moderate." — Meek, 

 1876. 1 



Placenticeras placenta (DeKay) Meek 

 Plate XII A 



Ammonites placenta DeKay, 1828, Ann. N. Y. Lye. Nat. Hist., vol. ii, p. 278, 



pi. v, fig. 2. 

 Ammonites placenta Morton, 1830, Am. Jour. Sci., 1st ser., vol. xvii, p. 279; 



vol. xviii, pi. ii, figs. 1-3. 

 Ammonites placenta Morton, 1830, Jour. Acad. Nat. Sci., Phila., 1st ser., 



vol. vi, p. 195, pi. v, fig. 4. 

 Ammonites placenta Morton, 1834, Syn. Org. Rem. Cret. Group, U. S., p. 



36, pi. ii, figs. 1, 2. 

 Ammonites placenta Meek, 1864, Check List Inv. Fossils, N. A., Cret. and 



Jur., p. 23 (in part). 



Etymology: Placenta, a flat cake; xepas, horn. 

 1 Rept. U. S. Geol. Survey, Terr., vol. ix, p. 463. 



