FORAMINIFEKA OF THE NORTH PACIFIC OCEAN. 



21 



25 and 26 is about 156°, that between succeeding chambers being 

 about 180°, so that chamber 25 is more than triloculine and less 

 than biloculine. In another of Schlumberger's figures the relation 

 of the change between triloculine and biloculine stages is still more 

 graphically shown, the angles of chambers after the quinqueloculine 

 stage measuring 124°, 123°, 150°, and 180°, respectively. The first 

 two are distinctly triloculine and the last one is distinctly biloculine, 

 the next to the last bridging over the gap almost exactly, the 150° 

 of the angle being exactly halfway between the ideal triloculine angle 

 of addition of chambers, 120°, and that of biloculine addition, 180°. 

 As a rule, the triloculine stage in Idalina seems to be short and fewer 

 in number of its chambers than either the preceding quinqueloculine 

 stage or the succeeding bilocu- 

 line stage, only three chambers 

 being usually arranged on the 

 triloculine plan. 



Following the triloculine stage 

 there is developed a definite bi- 

 loculine stage with chambers 

 added in planes 180° from one 

 another. In the specimen fig- 

 ured there are five such cham- 

 bers, 26 to 30, inclusive. Fol- 

 lowing the biloculine stage 

 chambers are added which com- 

 pletely encircle the previously 

 formed chambers so that but 

 one chamber is seen from the 

 exterior. This completes the 

 development and is a stage 

 higher than seen in living members of the Miliolidae except possibly 

 Nevillina coronata (Millett). 



A section of a specimen of Idalina antiqua with a megalospheric 

 proloculum is shown in figure 27. The proloculum is much larger 

 than in the microspheric form, but not as large as in certain other 

 specimens next considered. Following the proloculum is the Cornu- 

 spira-like second chamber. The third stage is represented by cham- 

 bers 3 to 10, inclusive, arranged on a quinqueloculine plan, in planes 

 144° apart as added. The fourth stage is formed by chambers 11 

 to 13, arranged on a triloculine plan, in planes 120° apart. Cham- 

 ber 14 initiates the biloculine stage, not developed in the micro- 

 spheric specimen until the twenty-seventh chamber. The unilocu- 

 line stage had not been attained in the specimen figured, the last- 

 formed chamber, No. 17, being still biloculine. 



Fig. 27.— Section of a megalospheric specimen of 

 Idalina antiqua. x 25 (after Munier-Chalmas 

 and schlumberger). quinqueloculine stage in 

 heavy shading, triloculine stage unshaded, 

 biloculine stage in light shading. 



