PLATE VIII. 



Orbiculina and Alveolina. 



Fig. 



1 — 5. — Successive stages of growth of the cyclical 

 type of Orbiculina adunca, showing the trans^i- 

 tion from the spiral to the cyclical plan of 

 growth : 16 diam. 



('>. — Horizontal section of a disk in which the cyclical 

 type has heen completely attained, the original 

 spiral being surrounded by a succession of 

 annuli : 16 diam, 



7 — 9. — Successive stages of growth of that type of 

 Orbiculina adunca in which the spiral plan 

 is maintained through life ; a, centre of the 

 spire, which forms the inner extremity of 

 every new zone ; c, its peripheral extremity ; 

 a b c, apertural surface : Ki diam. 



10. — Horizontal section of a large spiral example of 

 Orbiculina adunca : 16 diam. 



11. — Portion of the foregoing section much enlarged, 

 showing the chamberlets and their communica- 

 tions ; aa' , a a' , a a' , a a' , four zonal partitions, 

 perforated by the transverse passages b, b' , 

 which communicate between the successive 

 galleries c c , c c' , c c' , that connect the 

 rows of chamberlets d, d, d, which are divided 

 by the transverse partitions e, e, e, e : 100 

 diam. 



1^. — Vertical section of disk of Orbiculina adunca, 

 passing through its centre, 1, and showing the 

 mode in which this is invested by the suc- 

 cessive whorls, 2 2, 3, 3, 4, 4; a, multiple 

 zonal galleries between the columnar cham- 

 bers ; b, marginal openings of the radial 

 passages connecting successive zones : 100 

 diam. 



Fig. 

 13.- 



14.- 



15.- 



External aspect of Alveolina Quoii, showing its 

 longitudinal septal furrows, the spaces inter- 

 vening between which are crossed by secondary 

 furrows ; the elongated apertural plane is seen 

 to be perforated with rows of rounded pores, 

 of which those lying along the external margin 

 are smaller and more closely set than the rest ; 

 the apertural plane widens out greatly at its 

 two extremities, and the number of rows of 

 pores is greatly augmented, but they are less 

 regularly arranged : 40 diam. 



-Transverse section of Alveolina Qtioii, showing 

 the division of the spire into principal seg- 

 ments indicated by the inflection of the super- 

 ficial laminae at a, a, a ; each of these seg- 

 ments is divided by the lamina; d, d^, d~ 

 into a series of superposed chamberlets, 

 e, e', e', e", which open at their two extremities 

 into vertical spaces /, /, that extend through 

 the entire depth of the segment ; and in each 

 of these spaces are seen the orifices b, c, of 

 two galleries which pass along the entire 

 length of the shell, and connect the vertical 

 spaces with each other laterally : 80 diam. 



-Longitudinal section of Alveolina Quoii, showing 

 the mode in which the spire is arranged 

 around the primordial chamber, and in which, 

 by the irregular multiplication of chamberlets 

 at the two extremities, its length is augmented 

 much more rapidly than its diameter. Each 

 convolution of the solid spire is seen to be 

 perforated along its external margin by a 

 closely set row of minute pores, which are the 

 transverse sections of the long, narrow, super- 

 ficial chamberlets ; and beneath these are 

 several rows of larger and less approximated 

 pores, which are the transverse sections of the 

 subjacent chamberlets : 40 diam. 



