EXOGYKA PONDEROSA EEKATICOSTATA. 49 



I". S. Geol. Survey coll. 6455a. — Cut of Mobile & Ohio Railroad south of station at Booneville, Prentiss County, 

 Miss. Base of Selma chalk of northern Mississippi. (Within 50 to 100 feet of top of Exogyra ponderosa zone.) 



U. S. Geol. Survey coll. 6457. — Bald spot near Geevilleroad, 1A miles southwest of Booneville, Prentiss County, 

 Miss. Selma chalk, near top of Exogyra ponderosa zone. 



U. S. Geol. Survey coll. 6928. — Roadside exposure, one-quarter mile west of Adamsville, McNairy County, Term. 

 Impure phase of Selma chalk. (Near top of Exogyra ponderosa zone.) 



In North Carolina the species has been collected from 4 localities as follows: 



U. S. Geol. Survey colls. 4144, 5347. — Blue Banks Landing, Tar River, 7 miles above Greenville, Pitt County. 

 Black Creek formation. 



IT. S. Geol. Survey colls. 785, 5348. — Snow Hill, Greene County. Black Creek formation. 



U. S. Geol. Survey coll. 4142. — Contentnea Creek, 6 miles below Snow Hill, Greene County. Black Creek 

 formation. 



U. S. Geol. Survey coll. 5353. — Auger Hole Landing, Lenoir County. Black Creek formation. 



Exogyra ponderosa var. erraticostata var. nov. 



Plate XV, figure 4; Plate XVI, figures 1, 2. 



Description. — In all its characters except the surface ornamentation of the left or lower 

 valve this variety is essentially like the typical Exogyra -ponderosa Roemer. 



The surface of the left valve is characterized by the presence of fairly well defined, sharp 

 to rounded, ridged, radiating costse or plications which differ from the costse on Exogyra costata 

 Say in their generally weaker development and in their striking irregularity as regards size, 

 shape, and distribution (PI. XV, fig. 4). In proximity to the beak the shell is usually orna- 

 mented with small, regularly arranged costse (such as are present on some specimens of Exogyra 

 ponderosa Koemer, PI. XIII, fig. 5), extending backward over the shell one-half to three-fourths 

 of an inch (PL XVI, fig. 1) and merging into the irregular costse just described, which charac- 

 terize the variety. The irregular costse extend backward 3 to 5 inches from the beak, becoming 

 weaker in the direction of the margin ; in the larger individuals there is usually a considerable 

 part of the surface bordering the margin on which the costse are either very faint or are entirely 

 absent (PI. XVI, fig. 1). 



Remarks. — That this form is closely related to Exogyra ponderosa Roemer is proved by the 

 fact that practically all gradations are found between specimens t3~pical of the species, and the 

 extremes of the variety. The specimen illustrated in Plate XVI, figure 1, represents one of the 

 intermediate gradations; hi this specimen the irregular costa? are rather strongly developed in 

 front of the umbonal ridge on a part of the shell not showing in the illustration, for which reason 

 it is referred to this variety. 



Geologic occurrence.— In the eastern Gulf region Exogyra ponderosa var. erraticostata. appears 

 to be coextensive in its occurrence both stratigraphically and geographically with Exogyra pon- 

 derosa Roemer. It is present in the Tombigbee sand member of the Eutaw formation and 

 ranges upward to about the middle of the Selma chalk, where the latter is most fully developed 

 in western Alabama and east-central Mississippi, and is present in the corresponding nonchalky 

 equivalents of this part of the Selma chalk in eastern Alabama and Georgia — that is, hi approxi- 

 mately the lower one- third or one-half of the Ripley formation. The lower Jimit of its range is 

 shown in Plates IX and X by the dotted line indicated by the red letter P and the upper limit 

 by the red line 2. In the Carolinas the variety occurs in the marine invertebrate-bearfrig beds 

 forming the upper 'part of the Black Creek formation, and it is present questionably in the 

 extreme basal beds of the overlying Peedee sand. 



From Weller's description ' of Exogyra ponderosa Roemer the variety is doubtless present 

 in the Marshalltown clay marl of the Matawan group in association "with t.he typical forms of 

 the species. 



In Arkansas and northeastern Texas the variety occurs in the Brownstown marl. Farther 

 south in Texas it occurs in the basal part of tlie Taylor marl and it has been recognized in 



1 Weller, Stuart, The Cretaceous paleontology of New Jersey: New Jersey Geol. Survey, Paleontology, vol. i, 1907, text, pp. 16CM62. 

 105°— No. 81—14 4 



