FORAMINIFEEA OF THE NORTH PACIFIC OCEAN. 



17 



Fig. 21.— Chambers of microspheric 

 specimen of triloculina sciireiber- 

 iana d'orbigny (adapted from 

 sciilumberger). x 33. quinquelocu- 

 line chambers are shaded in the 



FIGURE. 



while in figure 20, the megalospheric, four such chambers exist, yet 

 the two specimens are of about the same diameter. 



There is a tendency, especially in the microspheric form, to build 

 a floor in the later chambers. In this specimen this character is 

 taken on in the twelfth chamber and con- 

 tinued throughout the further growth. 

 Development of Triloculina. — A sec- 

 tion of the test of Triloculina schrci- 

 beriana d'Orbigny with a microspheric 

 proloculum is shown in figure 2 1 . There 

 is the usual form of proloculum followed 

 by a Cornuspira-]ike second chamber. 

 Chambers 3 to 10 are arranged on a 

 quinqueloculine plan and the test is like 

 Quinqueloculina at this stage. The 

 chambers lie in planes 72° apart, but 

 successively added chambers 144° from 

 one another as in Quinqueloculina. Later 

 chambers, 11 to 16, are arranged on a 

 different plan, each chamber being 

 added in a plane 120° from its preced- 

 ing one, giving a test with but three 

 chambers visible on the surface. The method of building here is a 

 series of chambers made»up of single cycles, in each of which each 

 chamber lies on the outer side of its third preceding chamber instead 

 of the fifth, as in Quinqueloculina. This method of growth may be 

 called triloculine from the generic name Trilo- 

 culina, this character being typical of the adult 

 of that genus. The stages made out in the micro- 

 spheric form of Triloculina are four — (1) prolocu- 

 lum, (2) Cornuspira-lika chamber, (3) quinque- 

 loculine chambers, (4) triloculine chambers. 



A section of megalospheric specimen of this 

 same species is shown in figure 22. Here there 

 is a large proloculum followed by a second cham- 

 ber cut in two at opposite points in the section. 

 The following three chambers, 3 to 5, are some- 

 what irregularly placed, but the number of degrees 

 covered is about 275°, which gives an average of 

 about 138° for each angle. This is closed to the 

 144° of a typical Quinqueloculina, and these three chambers represent 

 the quinqueloculine stage in the development. These chambers, 3 to 5, 

 form a reduced quinqueloculine stage, which does not even form a com- 

 pleteseries. This is muchmore accelerated than the microspheric speci- 

 men, where there were eight chambers built on the quinqueloculine plan. 

 Chambers 6 to 9 are arranged in a triloculine plan, being added in 



Fio. 22.— Early chambers 



OF MEGALOSPHERIC SPECI- 

 MEN of Triloculina 



SCHREIBERIANA D'ORBIGNY 

 (ADAPTED FROM SCHLUM- 

 BERGER). X 66. 



