Mesozoic and Ccenozoic Geology and Paloiontology. 71 



generally' highly fossiliferous, and irregular, rounded nodules of iron 

 P3'rites of a radiated structure called "sulphur balls" are common. 

 It is of great thickness on its southwestern border in Chickasaw^ 

 Octibbeha, Noxubee and Kemper counties, where borings have been 

 made in it from 700 to 1000 feet, but there is a gradual thinning out 

 northward, through Pontotoc, Itawamba, Tippah and Tishamingo 

 counties to the line of Tennessee. The surface area of this sub- 

 division in Mississippi is greater than that of the other three com- 

 bined. 



The Ripley Group forms the border of the western exposure of the 

 Cretaceous, from a point in Chickasaw through the central part of 

 Pontotoc, the eastern part of Tippah and western part of Tishamingo 

 to the south line of Tennessee. It consists of hard crystalline lime- 

 stone, more or less sandy and glauconitic, which forms the highest 

 strata; and bluish micaceous marls, more or less sandy, and often in- 

 terstratified with subordinate ledges of sandy limestone, which latter 

 become less and less frequent as we descend in the series toward the 

 strata forming a transition into the Rotten limestone. 



Meek and Hayden* described, from (Fort Benton Group) the mouth 

 of Vermilion river. Ammonites vermilionemis, now Ifortoniceras ver- 

 niilionense ; from near the Black Hills, ScajJhites warreni ; from Little 

 Blue river, Inoceramus aviculoides, now J. prohlematicus, var. avicu- 

 loides ; from (Fox Hills Group), Moreau river, PhylloteHthis siibova- 

 tus, Dentalium pauperculmn, now Entdlis paupercida, and Cylichna 

 scitula ; from 20 miles below the mouth of Cannon Ball river, Tellina 

 fonnosa, now Linearia formosa ; from the north branch of Chej'enne 

 river, Cyprina humilis, now Veniella humilis ; and from Long Lake^ 

 Avicula subgibbosa, now A. linguiformis, var. subgibbosa ; from the 

 mouth of Judith river (Judith river Group), Helix evansi, now 

 Hyalina evansi; from the mouth of Grand river, Sphcerium jilaimm, 

 8. recticardinale, Cyrena cytheriformis, now Corbiciila cytheriformis^^ 

 and Inoceramus subcompressus, now I. cripsi^ var. subcompressus ; 

 from (Fort Pierre Group) the head of the south branch of Cheyenne 

 River, Uelicoceras angulatum, now Heteroceras angulatum, Amnion 

 lies placenta, var.intercalaris; now Placenticevas placenta^ var. inter- 

 calaris, from the Yellow Stone river, Scapjhites nodosus^ var. ijlenm, 

 Aporrhais parva, now Anchura parva, A. sublaevis, and 3Iactra 

 gracilis: from Fort Clark, Teredo s ell if or m is ; from White river, 

 Inoceramus vamixemi, I. balchi; from Bijou Hill, Anomia subtrigon- 



'■' Proc Acad. Nat. Sci. 



