10 



BULLETIN 71, UNITED STATES NATIONAL MUSEUM. 



allies, mainly in its open coil, there being little if any covering of the 

 preceding whorls by involution. As a result the development is 

 easily studied. 



The development of Spiroloculina oicarinata d'Orbigny from the 

 Eocene of the Paris Basin of Vandancourt may be taken as typical 

 of an Eocene Spiroloculina (fig. 8). The first chamber consists of a 

 globose proloculum. The second is the ordinary close-coiled, tubular, 

 Cornuspira-like chamber with no floor as seen throughout the group. 

 In this case its length is a little more than a half coil. The shell 

 wall of these two chambers is thin and of uniform thickness as in 



the other genera already described. The 

 third chamber at its inception is more than 

 twice as high as the aperture of the preced- 

 ing one, but gradually diminishes in height 

 until at its own aperture it is hardly, if at 

 all, higher than the aperture of the preced- 

 ing chamber. In length it is a little more 

 than a complete coil. It initiates two dis- 

 tinctive features of the genus. One of these 

 is the tendency of the distal portion of the 

 chamber to break away from the close-coiled 

 condition and to change the axis of the 

 chamber to a tangential position. As a 

 result, a tubular neck is formed which is a 

 rather general character among the Eocene 

 species of Spiroloculina. Another character 

 which is initiated here is the broad flaring lip 

 at the aperture. This is also characteristic 

 of a majority of the species of this period. 

 This third chamber, therefore, initiates 

 several important characters, but not the 

 one of prime generic importance, the half 

 coil length of chambers in a single plane. 

 Beginning with the fourth chamber the 

 specimen is at once definitely a Spiroloculina. 

 The fifth and succeeding chambers simply add to the size of the in- 

 dividual or to the specific characters, no generic characters having 

 been attained in the fourth chamber. 



As the apertural lip when formed extends beyond the rest of the 

 test, the space thus left is filled in before the next chamber is built. 

 This is shown in the specimen figured, which was apparently about to 

 form a new chamber. This shelly growth which is filled in is usually 

 less solid than the rest and may appear of a lighter color when exam- 

 ined as a balsam mount with transmitted light. In some species 

 such as S. perforata this space is left open and the new chamber is 



Fig. 8.— SrmoLOCULiNA bicarin- 

 ata d'Orbigny. X85. Eocene 

 of Vandancourt, Paris Basin. 

 Specimen viewed by transmit- 

 ted light. (1) Proloculum, (2) 

 second cornuspiralike cham- 

 ber, (3) elongated chamber, 

 (4, 5) adult chambers a half 

 coh in length. 



