28 



CRETACEOUS DEPOSITS OF THE EASTERN GULF REGION. 



Mollusca — Continued. Mollusca — Continued. 



Gyrodes abyssina (Morton), a 6c. B. asper Morton, be. 



G. crenata Conrad, a be. Placenticeras planum Hyatt, be. 



Turritella trilira Conrad . a be. P. guadalupse (Roemer). 6c. 



T. quadrilira Johnson. 6c. Mortoniceras sp. (alt. M. texanum (Roemer) ). 6c. 



Nautilus sp. nov. (large). 6c. Hamites torquatus Morton. 6c. 

 Baculites anceps Lamarck? 5c. 



Of the species listed 37 are restricted to the Exogyra ponderosa zone; 10, or questionably 

 11, range downward into the basal beds of the Eutaw formation (hi the Chattahoochee region 

 only) ; and 58, or questionably 60, range upward into the overlying Exogyra costata zone. Two, 

 which occur in the zone of Exogyra costata, 'are questionably present in the Exogyra ponderosa 

 zone. 



Of the 37 restricted species 20 occur in the marine invertebrate-bearing beds forming the 

 upper part of the Black Creek formation of the Carolinas, and of these but 1, and that ques- 

 tionably, occurs hi the basal beds of the overlying Peedee sand of the same region; hi addition 

 1 Black Creek species is questionably identified from the Exogyra ponderosa zone. Of the 

 37 species 6 occur x in the Matawan group of New Jersey; and all 6 except the species Liopistha 

 alternata are common to the Black Creek formation of the Carolinas. The genus Mortoniceras, 

 which hi the eastern Gulf region occurs only hi the Mortoniceras subzone, is present in New 

 Jersey only in the Merchantville clay marl, which forms the basal formation of the Matawan 

 group. None of the 37 species have been reported from the overlying Monmouth group, and 

 but 1, Turritella quadrilira Johnson, from the underlying Magothy formation. 



SUBZONES OF THE EXOGYRA PONDEROSA ZONE. 

 OCCURRENCE OF THE FOSSILS. 



The zone of Exogyra ponderosa is divisible paleontologically into two parts — first, the 

 Mortoniceras subzone, which, from Chattahoochee River to Aberdeen, Miss., is regarded as 

 approximately coincident with the Tombigbee sand member of the Eutaw formation, the 

 chief occurrence of the genus Mortoniceras, however, being hi the upper 75 to 100 feet of that 

 member; and second, the remainder of the zone, or the part included between the Mortoniceras 

 subzone and the base of the overlying Exogyra costata zone. Localities on Chattahoochee 

 River and hi eastern Alabama have yielded the bulk of the species from both of these parts. 



The Tombigbee sand, where overlain by the Selma chalk, does not terminate abruptly in 

 its upper part, but passes by gradation through 5 to 10 feet of impure sandy limestone into 

 the overlying chalk rock. The species characteristic of the Mortoniceras subzone are not 

 restricted to the typical Tombigbee sand strata but range upward into these transition beds, 

 for which reason the latter are considered to belong to the Tombigbee sand member rather than 

 to the Selma chalk. In the region between Montgomery. Ala., and the vicinity of Hamburg, 

 in Perry County. Ala., collections have been made at numerous places in fields where the fossils 

 have weathered from these transition beds. For the purpose of this discussion these collections 

 have been considered as coming from the Mortoniceras subzone, although there is admittedly 

 a chance for the intermixture of fossils weathered from slightly higher beds of the Selma chalk. 

 Portions of the matrix, however, adhere to many specimens, thus affording a means of judging 

 of the approximate stratigraphic positions of the bods from which they were weathered. The 

 possible error is in any event too slight to justify discarding the many valuable specimens 

 obtained from this source. 



1 this statement is made on the assumption that hocardia cliffwoodensis Weller is correctly identified, and that Lucinu glebula Conrad and 

 Lucinn cretacta Whitfield are synonymous. 



