THE FORAMINIFERA 



through the central chamber and the mouths of all the chambers 

 which succeed it, has been called the axis of construction. The 

 spiral formed by the series of chambers is not quite regular, as is 

 the case in Cornuspira ; for while each chamber is gently curved, 

 there is a sharp bend where one chamber communicates with 

 another. Hence the test is elongated in the axis of construction. 

 In Spiroloculina the chambers are disposed in one plane, and the 

 width of each is only slightly greater than that of its predecessor, 

 so that all the chambers are exposed on the two flat faces of the 

 test (Fig. 21). 



In all but the earlier chambers of the microspheric forms the 

 arrangement characteristic of the genus Biloculina is essentially 

 similar, but there is a marked difference in the shape and appear"- 

 ance of the test owing to the great width of the chambers. Each 



Fio. 22. 



a, Biloculina dcpressa, rt'Orb., x 40. b, 

 Triloeulina tricarinata, d'Orb., x 50. (After 

 Brady, 3.) 



is so wide that its margins are in contact with those of its prede- 

 cessor, and overlap them at the sides (Figs. 22, a, and 24). It results 

 from this arrangement that the two last chambers enclose those 

 previously formed, and they alone appear in the contour of the 

 test. As in the preceding genus a median longitudinal section 

 through the last chamber divides the whole series of chambers 

 into symmetrical halves. As will appear later, the microspheric 

 form of Biloculina departs considerably from this arrangement. 



In Triloculina and Quinqueloculina 1 the chambers are likewise 

 disposed about an axis of construction, and their mouths open 

 alternately in opposite directions, but the median plane of any 

 chamber is not that of its predecessor, but directed at a definite 

 angle to it. It is as though in a Biloculina test, while the plane 

 in which the new chambers are formed remains constant, the 



1 These genera are now usually included in the genus MUiolina, though Schlum- 

 berger is inclined to retain the old generic distinctions. 



