STEUCTUEE AND EEPEODUCTION 29 



chambers is bilocnline from the commencement, hut 

 in form B the minute central chamber is followed by 

 a series of chambers arranged in the quinquelocu- 

 line manner, the biloculine arrangement not being- 

 attained until the eleventh chamber is formed. 

 Many other genera were described by these authors 

 as having true dimorphic characters. 



In the case of OrhitoUtes covipJdiiata, which has 

 been investigated by Brady and Lister, its dimor- 

 phism presents many points of interest. Porm A is 

 smaller than form B ; and Lister found that in a 

 large series of the former the number of rings of 

 chamberlets is usually under thirt}', whilst form B 

 has from 79 to 110 rings. As the microspheric form 

 increases in size the edge of the disc becomes 

 double, so that a radial section at right angles to 

 the plane of the disc is Y-^l^^P^^- The marginal 

 chambers of this form (B) are not divided into small 

 chamberlets, but consist of large chambers as high 

 as the disc is thick. These brood-chambers, as they 

 are called, contain the young forms, which in all 

 cases are megalospheric. The megalospheric forms, 

 on the other hand, often show numerous compact 

 bodies of different sizes and irregular shape, derived 

 by disintegration of the nucleus. 



The megalospheric form is, as a rule, much more 

 abundant than the microspheric. Of PoJijstomella 

 crista Lister gives the proportion which he found 

 of forms A and B as 34 to 1. Of Adelosina l^oly- 

 gonici Schlumberger gives the proportion as 8 to 1. 



