TRIMORPHISM 359 



Another example may be seen in Idalina antiqua (d'Orbigny). 

 In the section of a specimen with a microspheric proloculum, 

 there are 15 chambers developed after the proloculum. The 

 early chambers I-VIII are quinqueloculine, IX-XII triloculine, 

 and the remainder biloculine, the normal sequence in micro- 

 spheric forms of this group. Fig. 2 is a section of a specimen 

 with a small megalospheric proloculum and having but 9 cham- 

 bers after the proloculum. There is no truly quinqueloculine 

 stage, chambers I-VI showing the triloculine stage and the re- 

 mainder biloculine. In the figure is a section of a specimen with 

 a very large megalospheric proloculum and with but 4 chambers 

 after it, all of them biloculine. If this were not an involute 

 species covering the early stages an exactly parallel case would 

 obtain to that previously given. Graphically the stages may be 

 represented as follows : 



P+Q+T+B for the microspheric form. 



P+T-f B for that with a smaller megalospheric prolocu- 

 lum. 

 P+B for that with a larger megalospheric proloculum. 



Hofker in his work on the Foraminifera of the Siboga Expe- 

 dition gives many examples along this same line, and they are 

 well known to all students who have worked with many forms. 

 There are many species of Quinqueloculina in which the micro- 

 spheric and megalospheric forms are known and in which no de- 

 velopment goes beyond the quinqueloculine stage. Quinquelocu- 

 lina then is a good genus for such forms. There are many 

 species of Triloculina in which the microspheric form has a 

 quinqueloculine young and the larger megalospheric forms do 

 not go beyond a triloculine stage. For all such forms, Trilocu- 

 lina becomes a good genus. Biloculina may have all three forms : 

 the microspheric quinqueloculine, then triloculine, and finally 

 biloculine; the form with the smaller megalospheric proloculum 

 triloculine then biloculine and lastly individuals with a very 

 large megalospheric proloculum that become at once biloculine. 

 For all these, Pyrgo (Biloculina) becomes a perfectly good genus. 



So in the case of other groups if all three forms seem to show 

 different generic characters, there is diflficulty, but if one gen- 

 eric character is involved, the generic problem is not difficult. 

 It may be shown that when all three forms occur without the 



