12 PALAEONTOLOGY 



Lagena, with the tubular neck inverted into the cavity of the 

 shell 



Among the many-chambered Foraminifers the modifica- 

 tions of form seem endless. Nodosaria resembles a cylin- 

 drical beaded rod : Cristellaria begins by being spiral and 

 afterwards becomes straight : most species are wholly spiral : 

 in some, as Nwmn/idites, the convolutions are on the same 

 plane : in many the spiral turns obliquely round an axis, and 

 gives the shell a trochoid form. 



Upwards of six hundred and fifty-seven fossil species, 

 belonging to seventy-three genera, have been described : they 

 commence in the paheozoic age, increase in number and 

 variety with each successive stratum, and attain their maxi- 

 mum in the present seas. Most of the fossil genera, and 

 even some of the species, pass through many formations ; 

 indeed, if correctly observed, the existing forms are the oldest 

 known living organisms. Dcntalina communis, Orbttolites 

 coinplanatus, Roscdina itcdica, and Rotalina globvlosa, all living 

 species, are said to be found in the chalk; Rotalina wmJbv- 

 licata ranges to the gault ; and Wcbbina rugosa is common to 

 the upper lias, the chalk, and present sea. It has, however, 

 been observed, that fossil Ehizopods, set free by the disinte- 

 gration of rocks, are mingled with the recent shells on eveiy 

 beach ; and Mr. M' Andrew has obtained them in this condi- 

 tion from great depths of the mid-channel. 



The earliest important form is the FustUina (fig. 2, 5), 

 which forms layers many inches, or even feet, in thickness in 

 the carboniferous limestone of Russia. The recent genera 

 Dcntalina and Textularia are found in the magnesian lime- 

 stone ; Xodoxaria, Cristillaria, and Rotalia, in the lias. Fla- 

 bellina (fig. 2, 6) is peculiar to the chalk ; Orbttoides (fig. 2, 9) 

 to the chalk and tertiary series ; Ovuliks (fig. 2, 10) is peculiar 

 to the eocene tertiaries; Operoidina, Orbttolites, and Alveolina 

 appeal' first in the tertiary, and are still Living. The Lituola 



