FORAMINIFERA OF THE NORTH PACIFIC OCEAN. 19 



bers arranged on quinqueloculine and triloculine planes followed in the 

 adult by chambers arranged on bilocnline planes. A microsperic 

 specimen of this species of Biloculina repeats in its own ontogeny 

 characters seen in the adults of the more primitive types, Cornuspira, 

 Quinqueloculina, and Triloculina in the order in which they were 

 developed, as has already been noted in the ontogeny of those genera. 

 In specimens of various species of Biloculina where there is a 

 megalospheric proloculum there is an accompanying acceleration of 

 development as shown in figure 24, winch is a section of B. ringens 

 of the megalospheric form. A proloculum and Cornuspira-like 

 second chamber form the first and second stages. Next, instead of 

 a quinqueloculine stage, a triloculine stage is at once taken on by 

 chamber 3 and continued through chamber 10. The angle between 

 the chambers is rather constant, averaging 120°, except that be- 

 tween chambers 9 and 10 where the change from the 120° of trilocu- 

 line to the 180° of biloculine growth occurs. There the angle is a 

 medium between these two 

 as is also that between 

 chambers 10 and 11. The 

 biloculine stage starts defi- 

 nitely with chamber 11. 

 This megalospheric speci- 

 men is then more acceler- 

 ated than the microspheric, 

 skipping the early quinque- 

 loculine stage and taking 

 on the adult biloculine plan 



Of growth in Chamber 11, FlG ^megalospheric specimen of Biloculina mur- 



not reached in the micro- rhyna, with the biloculine stage begun at once, the 

 spheric specimen until the 7™™™™^^™ '"~ "'""* m 

 fifteenth chamber. 



A section of a specimen of Biloculina murrhyna with an excep- 

 tionally large megalospheric proloculum is shown in figure 25. The 

 proloculum is very large and the Cornuspira-like second chamber 

 small in comparison. Chambers 3 and 4, instead of forming the 

 beginning of a quinqueloculine stage or even the triloculine stage as 

 in the preceding here begin directly upon the biloculine character, 

 the quinqueloculine and triloculine stages having been entirely 

 skipped. 



Development of Idalina. — The genus NeviUina as shown hi the fig- 

 ures on plate 35, after Sidebottom, shows that we have here a genus 

 in the present seas going a stage beyond Biloculina and developing 

 almost a single chamber externally in the adult. In the fossils, 

 however, even better examples occur. In Idalina, a genus from the 

 Upper Cretaceous of Southern Europe, the highest type of devel- 



