PLATE XVIII. 



All the figures in this Plate illustrate the structure of Nmnmulina ; and the same letters indicate corresponding 



parts throughout. 



a, a. Marginal cord, as seen in section through the median plane ; a', a', the marginal cord, as seen in 



sections dividing it transversely ; a", furrowed surface of the marginal cord. 

 h, h. Septa dividing the chambers of the median plane; b' , h' , prolongations of those septa over the 



lateral surface of the included whorl; b" , b" , bifurcating prolongations. 

 c. Apertural fissure. 



d, d. Spiral lamina enclosing the chambers of the median plane ; d', d' , investing layers of the spiral lamina. 



e, e. Columns of non-t\ibular substance, divided longitudinally by sections at right angles to the median 



plane ; e , e , the same divided by transverse sections. 

 /, /. luterseptal canals, as seen traversing the septa longitudinally ; /, /', the same, as seen in transverse 

 section. 



Fig. 



1. — Portion of A^. gi:ehensis, showing the alar pro- 

 longations of the marginal chambers over the 

 included whorl. (After D'Archiac.) 



2. — Portion of a vertical section of N. Itevigata, 

 showing the tubular structure of the spiral 

 lamina, with non-tubular columns interposed, 

 the canal-system of the marginal cord as seen in 

 a transverse section, and the apertural fissure. 



3. — Portion of N. garansensis, showing the reticular 

 disposition of the alar prolongations of its 

 septa, forming numerous isolated chamberlets. 

 (After D'Archiac.) 



4. — Small portion of a thin section of N. Icevigata 

 through the median plane, showing the distri- 

 bution of the canal-system in the septa and 

 marginal cord. 



5. — Larger portion of the same section, less highly 

 magnified, showing the general disposition of 

 the spiral convolutions. 



6. — Portion of another section taken parallel to the 

 median plane, highly magnified, showing the 

 tubular structure of the spiral lamina, the 

 transparent, non-tubular structure of the septal 

 bands, and the distribution of the canals in 

 the septa and marginal cord. 



7. — Portion of N. lavigata laid open by a vertical 



Fig. 



fracture, showing the furrowed surface of the 

 marginal cord of the included whorl, and parts of 

 two large spiral canals, from which proceed the 

 interseptal canals that pass towards the mar- 

 ginal cord of the outer convolution. 

 8. — Portion of N. Icevigata as laid open by vertical 

 fracture in the direction transverse to the 

 radius of the disk, showing the interseptal 

 canals transversely divided, and the columns 

 of non-tubular substance commencing on the 

 septa of the median layer, and extending to 

 the surface through the successive layers of 

 the spiral lamina. 

 9. — Portion of A^. kevigaia, showing the sub-reticular 

 disposition of the alar prolongations of the 

 septa. 



10. — Vertical section of N. lavigaia, showing the 

 septa and apertural fissures of the chambers 

 of the median plane, the continuity of the 

 successive layers of the spiral lamina over 

 both lateral surfaces, the interposition of the 

 alar prolongations of the chambers, with their 

 dividing septa between those layers, and the 

 columns of non-tubular substance which pass 

 sometimes through one or two of them, some- 

 times through several. 



