78 



FAMILY MILIOLIDA. 



however, that the alar lobes of one side are more prolonged than those of the other, so that 

 more of the earlier chambers of the shell are shown on the one face than on the other ; in this 



Fig. XVII. 

 A. 



B. 



C. 



Ideal transverse sections of Bilocnlina. 



Ideal transverse seel ions of Qiii».queloculina. 



way it often comes to pass that fice chambers are visible externally on one side of the shell, 

 whilst only three are apparent on the other (Fig. XVII) ; and this type is the Quinqudoculina of 

 D'Orbigny (Plate VI, figs. 3 — G). It is, however, subject to great inconstancy as to the 

 number of chambers visible externally ; this being usually from fJiree to citjld on one side, and 

 from lico to (six on the other ; some shells formed on this plan having only tivo chambers visible 

 on one side and three on the other.* A greater departure from the ordinary mode of growth, 

 however, is shown by those Miliolm in which the alternation in the direction of the successive 

 segments of the sarcode-body is not duplex but triplex ; so that the chambers of the shell, 

 instead of lying on the two sides of the diameter of the spire, cluster around it (so to speak) 

 triangularly, as shown in Fig. XVIII ; thus causing the shell to become three-sided, and ren- 



FiG. XVIII. 



Ideal transverse sections of Triloculina. 



* It was from having perceived the unimportance of these numerical variations in shells formed 

 on the plan above described, that Profs. Schultze (xcvii) and Williamson (ex) were led to reunite 



