8 University of Texas Bulletin 



folds and developing fine ribs, but retaining the high um- 

 bonal ridge. 



In the same strata we find another species, of which a 

 specimen not very characteristic has been illustrated by 

 Cragin in his fig. 4. The upper part (about 15 mm. in 

 height) is practically of the same form and ornamentation 

 as Ex. plexa typ. but from there on it begins to develop 

 on the anterior side four to five strong radial folds begin- 

 ning near the umbonal ridge and going down obliquely with 

 the ridge to the antero-inferior margin. They are much 

 stronger and sharper than shown in Cragin's figure and are 

 covered by fine radial ribs. They are fainter on the mould 

 and there appear smooth. As our material is not sufficient 

 to allow a complete description we shall designate the species 

 as Ex. nov. sp. aff. plexa. (PI. 1, fig. 5). 



Ex. plexa seems to lose the fine ribs on the lower margin 

 in larger specimens, as is shown by an individual from the 

 original locality. This feature is still more evident in larger 

 specimens found at the base of the Fort Worth limestone in 

 Duck Creek, Grayson County. This is evidently a younger 

 form of the same group, but possibly a different spedes or 

 variety. It grows much larger than the type, shows ap- 

 proximately the same ornamentation on the umbonal region, 

 but becomes entirely smooth on the inferior margin. 



Another form from the Kiamitia clays found on little 

 Mineral Creek, north of Pottsboro, Grayson County, shows 

 an ornamentation very similar to that of Ex. plexa on the 

 umbonal region, but develops a rather deep furrow along 

 the umbonal ridge on the posterior side and several obtuse 

 folds covered with fine radial ribs on the anterior side near 

 the antero-inferior margin. It is most probably a descend- 

 ant of Ex. nov. sp. aff. plexa, mentioned above (PI. 1, fig. 6.) 



About the development of Exogyra in the Georgetown 

 formation, i. e., the lower and middle part of the Cenoman- 

 ian, little is known. 



There is a large species frequently found in this forma- 

 tion and it has generally been called Ex. americana or Ex. 

 Walkeri. It was originally described by Marcou as Gry- 

 phaea sinuata var. americana, and Cragin cites it as Ex. 



