16 



BULLETIN 11, UNITED STATES NATIONAL MUSEUM. 



little more than a half coil in length. This chamber is shown cut 

 once on the left side of the proloculum. The succeeding chambers, 

 3 to 13, are arranged on a quinqueloculine plan of growth, the cham- 

 bers as added being in planes 144° from one another, five chambers 

 thus appearing on the periphery in planes 72° from one another. 

 In the adult, figure 18 of the same microspheric form, the quinque- 

 loculine growth is seen to continue to the seventeenth chamber. 

 The eighteenth and nineteenth chambers, instead of continuing the 

 quinqueloculine plan of growth, are placed in planes nearly 180° 

 from one another and initiate the adult character, that of cham- 

 bers in an open coil lying in one plane. 



A young individual of the megalospheric form is shown in figure 

 19. Without the last developed chamber, No 12, it would be taken 

 for a young Quinqueloculina. There is a large proloculum followed 

 by a second chamber of the usual Cornusjrira-like form. Chambers 

 3 to 11 are arranged on the typical quinqueloculine plan as in the 

 microspheric form already noted. Chamber 12, however, is added 

 in a plane nearly 180° and from chamber 1 1 initiates the adult charac- 



FiG3. 19, 20.— Massilina secans (d'Orbigny). Megalospheric form (adapted from Schlumberger). 

 19, Young specimen still in quinqueloculine stage, x 20. 20, Adult specimen. X 13. 



ter. This specimen is then more accelerated than the microspheric 

 specimen, figure 18, as it takes on this character several chambers 

 earlier than did the microspheric. An adult specimen of the mega- 

 lospheric form showing greater acceleration is shown in figure 20. 

 Here the proloculum is nearly twice as large in the previous megalo- 

 spheric specimen. Chambers 3 to 7 are arranged on the quinque- 

 loculine plan, but chamber 8 is added in a plane 180° from chamber 

 7, thus initiating the adult character very early. This character 

 did not appear in the other megalospheric specimen until the twelfth 

 chamber and in the microspheric until the eighteenth chamber; 

 chambers 8 to 11 are in an open coil in one plane, completing the 

 development, while in both of the other specimens it was still in the 

 quinqueloculine stage in the eleventh chamber. 



As noted in other species, the microspheric form attains the largest 

 size, as seen by comparing the two figures, figure 18 and figure 20, 

 which are drawn with the same magnification. In the microspheric 

 form, figure 18, there have been but two Massilina chambers built, 



