18 



BULLETIN 71, UNITED STATES NATIONAL MUSEUM. 



Fig. 23.— Biloculina ringens. Early chambers 

 of microspheric form (adapted from schlum- 

 berger). x 50. the early quinqueloculine 

 chambers are shown with dark shading. 

 The later triloculine stage unshaded and 

 the final biloculine chambers in a light 



SHADING. 



planes 120° from one another. The megalospheric form is thus more 

 accelerated, taking on the triloculine character in the sixth chamber 

 a character not initiated in the microspheric specimen until the 

 eleventh chamber. 



It will then be seen that the megalospheric form completed its 

 growth with the ninth chamber, but the microspheric specimen in its 



ninth had not even attained the 

 adult triloculine character. In 

 matter of size the megalospheric 

 specimen, with its eighth chamber, 

 had built a test of about the same 

 size as the microspheric specimen 

 "with 16 chambers. 



Development of Biloculina. — A 

 section of Biloculina ringens in the 

 horizontal plane is shown in figure 

 23. This specimen has a micro- 

 spheric proloculum followed by 

 a Cornusinra-hke second chamber 

 of the usual sort in this group, a 

 half coil in length thus being cut 

 at but one point in the section. 

 Chamber 3 starts a quinqueloculine 

 stage continued through chamber 8 of the usual arrangement seen 

 in Quinqueloculina, chambers 72° apart but successively added in 

 radial planes 144° apart, as shown in the adult of Quinqueloculina 

 and in the young of Triloculina. Chamber 9 initiates the triloculine 

 stage, and with chambers 10 and 

 1 1 completes a cycle of chambers 

 120° apart. This triloculine stage 

 is here continued through cham- 

 ber 14. Chamber 15 initiates 

 the biloculine stage where cham- 

 bers are added in planes 180° 

 from one another, the newly 

 added chamber covering all ex- 

 cept the last previously formed 

 chamber. Thus in typical Bi- 

 loculina but two chambers are 

 visible from the exterior. Fur- 

 ther growth is an increase in size 

 by the addition of new chambers 

 on this biloculine plan, the number of these chambers depending 

 largely upon the age of the individual. 



In its microspheric form, then, Biloculina develops a proloculum 

 and Cornuspira-like second chamber, followed successively by cham- 



FlG. 24.— MEGALOSPHT2RIC SPECIMEN OF BILOCULINA 

 RINGENS. X 80 (ADAPTED FROM SCHLUMBERGER). 



