PLATE V. 



This Plate is cliiefly devoted to illustrations of the varieties of form and development presented by the 

 Genera Nubecularia and Vertebralina. All the figures of each genus are drawn under the same magnifying 

 power, to show the remarkable variation which they exhibit in the size of their component parts. 



Fig. 

 1. — Interior of a specimen o( Nubecularia m which 

 the spiral mode of growth continues with 

 unusual regularity. (See Fig. 6.) 



2. — Interior of a specimen in which the regular 

 spire has very early given place to a succession 

 of large, irregularly-disposed chambers, 



3. — Interior of a specimen in which the regular 

 spire has early given place to an assemblage 

 of chambers arranged without any succes- 

 sional regularity whatever. 



4. — Under surface of a specimen in which the 

 dimensions of the chambers, which follow one 

 another in a regular spiral for about two 

 turns, are at first unusually small, and then 

 suddenly increase. 



5. — Interior of a specimen in which the chambers 

 are unusually large from the commencement. 



n. — External surface of a specimen resembling that 

 of which the interior is shown in Fig. 1. 



7. — Interior of a specimen in which the original 

 spiral has very early given place to a rectilineal 

 series of very large chambers, the highest of 

 which seems not only to have continued the 

 series in nearly the same direction, but also to 

 have given forth an elongated chamber, which 

 doubles back (on the left side of the figure) 

 along the line of the preceding chambers. 



S. — Exterior of a specimen formed by the acervuline 

 clustering of chambers around the horny stem 

 of a Zoophyte. 



9. — Exterior of a specimen formed by the acervuline 

 clustering of chambers around a small branch 

 of Isis hippuris. 



10. — Interior of part of a specimen in which a uni- 

 serial plan of growth is giving place to a more 

 extended arrangement ; the lowest chamber 

 opens into the next by a single aperture, but 

 the succeeding aperture is divided into two, 

 both leading to one chamber, the aperture of 

 which again is single. This chamber, how- 

 ever, opens by two apertures into two separate 

 chambers, each of which seems from its direc- 

 tion to have given origin to a distinct series. 



13 



14 



Fig. 



11. — Under side of a specimen in which a spiral suc- 

 cession of very small chambers is maintained 

 with great regularity for several turns ; near 

 its margin the chambers show a disposition to 

 become elongated, as in Fig. 13, in which case 

 the spiral would probably give place to a 

 cyclical plan of growth. 



12. — Interior of a specimen in which a nniserial is 

 converted into a biserial arrangement of cham- 

 bers, apparently in accordance with the branch- 

 ing of the stem of a Zoophyte, probably an 

 Isis, to which it was attached. 



Exterior of a specimen growing on the surface 

 of a flat shell, in which the chambers are ex- 

 traordinarily extended laterally. 



Under side of a similar specimen, showing the 

 manner in which the original spiral plan of 

 growth soon gives place to another, in which 

 a series of chambers, extended laterally (as in 

 the preceding figure), communicate with each 

 other by multiple orifices. 



15. — Interior of a specimen which connects such 

 aberrant forms with the more general type, 

 the chambers not departing much from the 

 form they usually have in the nniserial arrange- 

 ment, but communicating with each other by 

 multiple orifices instead of by a single wide 

 aperture. 



16. — CortiKspira foliacea. (After Williamson.) 



17. — Small milioloid variety of Vertebralina striata, 

 from the Tertiary sand of Baltjik. 



18. — Renuline variety of Vertebralina striata, from 

 the Eocene at Hauteville. 



19. — Articuline variety of Vertebralina striata, from 

 the Tertiary sand of Baltjik. 



20. — Arrested variety of Vertebralina striata (the V. 

 cassis of D'Orbigny). 



21. — Irregular specimen of the typical form of Verte- 

 bralina striata. 



22. — Typical specimen of Vertebralina striata. 



23. — Stunted specimen of Vertebralina striata. 



24. — Dwarfed specimen of Vertebralina striata, from 

 a depth of 360 fathoms. 



25. — Broad, compressed specimen of Vertebralina 

 striata. 



