Structure of Ceratobulimina

Abstract

The large size and the abundance of tests of Ceratobulimina eximia (Rzehak) in Claiborne (middle Eocene) strata in central Texas offer excellent opportunity to observe the fundamental details of internal structure of tests of this genus cf Foraminifera. The delicacy of its minute features and the thinness of the shell matter composing them render observations somewhat difficult, since few specimens remain intact even after the most careful washing of samples containing them. Compact clays rich in this species have yielded many fresh and perfect tests, some of which have been carefully broken to reveal the inner structure of the chambers. After studying the robust Claiborne species, well-preserved specimens of C. cretacea Cushman and Harris of the Navarro formation and C. perplexa (Plummer) of the Midway group of strata were studied and found to carry the same structure. The final chamber of a ceratobulimine test is characterized by a thin wall, which on some specimens is so nearly transparent as to permit a view of the features of the inner structure without breaking away the outer wall. The apertural face is especially thin, and its lower half is characterized by a still thinner and faintly bulging area, at the end of which is the true aperture of the test. In Ceratobulimina eximia the long axis of the semi-lunar bulging area coincides with the long axis of the face (double-headed arrow, Fig. 7), and the aperture is a very narrow -slit along the base of the face (at base of bulging area). The slit lies over part of the ventral side of the previous whorl and cver part of the umbilicus, and it extends from a notch near the periphery (Fig. 7, a) to another notch on the edge of the umbilicus (Fig. 7, b). The notch near the periphery marks the ventral end of the line of attachment of the final chamber to the previous whorl of the test. The umbilical notch marks the lower end of a fold that anchors an inner chamber partition to the ventral side of the septal face. In C. cretacea and in C. perplexa the axis of the bulging area is almost at right angles to the long axis of the septal face and is directed into the umbilicus (arrow, Figs. 1, 5). The bulging area is bounded on one side by the line of attachment of the final chamber to the previous whorl and on the other by the strong umbilical notch and dent (Figs. 1, 5). The aperture of each of these two species lies, therefore, in the umbilicus and at the end of the bulging area. The septal face of each of the three fossil species under observation exhibits a distinct dent that extends from the notch in the edge of the umbilical depression diagonally across the lower and ventral side of the face and along one side of the bulging area. When the dorsal wall of the final chamber is broken away (Figs. 4, 9, 10), the sharp dent is found to mark the position of a fold that acts as the ventral attachment of a delicately serrate shelf or incomplete partition that hangs from the final septum below the transversely elliptical foramen and extends laterally across the septum to the dorsal suture, where it narrows to a point and where it shows externally on manv 460