FORAMINIFERA OF THE NORTH PACIFIC OCEAN. 



89 



Fig. 43.— Early chambers of Orbi- 

 culina compressa d'orbigny, x 83. 



Development o/OrbieuMna compressa d'Orbigny. 



The early chambers in a specimen of this species are shown in 

 figure 43. There is an oval prolociilnm with a thin wall of uniform 

 thickness. The second chamber is elongate, closely coiling on the 

 surface of the proloculum for about a half coil in length. Both the 

 proloculum and this second chamber are finely perforate, a character 

 seen also, as already noted, in the young of Peneroplis. The third 

 chamber is globular and connects with 

 chamber 2 by a single aperture. Cham- 

 bers 3 to 7 are of the same general char- 

 acter, and at this stage (in section at least) 

 this might be the young of Peneroplis. 

 Chamber 7 completes a single coil from 

 the proloculum. This number of cham- 

 bers is also very similar to the condition 

 seen in the first coil of Peneroplis. 

 Chamber 8 adds a new character, that of 

 multiple apertures, there being two in 

 this chamber. Chambers 8 to 12 each 

 have two apertures. Chamber 15 has 

 developed three apertures. 



As the chambers increase in height there 

 is a corresponding increase in the number of the apertures. In the 

 later development of this same specimen (fig. 44) the chambers in- 

 crease rapidly in height from the sixteenth to the twentieth. The 

 outer end of each chamber keeps to the outer curve of the test 

 which has taken on a less closely coiled condition. As the opposite 

 or inner end of each chamber extends back to the previous coil, a 



considerable increase in the extent of the 

 chamber must necessarity result. Chamber 

 21 initiates a new character, its outer end 

 extending beyond that of the preceding and 

 back on the outer curve of the test. This is 

 continued by the following chambers each 

 extending back at both ends to the wall of 

 the earlier built portion. This method of 

 growth is continued until the opposite ends 

 of a chamber meet on the peripheral wall 

 and make a complete annulus. This takes 

 place in this individual at the thirty-fifth 

 chamber. From this point each chamber forms a complete annulus 

 about the preceding chambers throughout succeeding growth. 



Development o/Orbiculina adunca (Fichtel and Moll). 



This species in its early development is in most respects compar- 

 able to that of the preceding species, 0. compressa, but is much more 

 53709°— Bull. 71, pt. 6—17 7 



Fig. 44.— D evelopment of a 



SPECIMEN OF ORBICULINA COM- 

 PRESSA d'Orbigny, up to and 



INCLUDING THE TWENTY-FOURTH 

 CHAMBER. X25. 



