EHIZOPODA. 13 



2. Rhabdoidea, in which they are placed in a direct line 



(Stichostega, d'Orb.) ; and 



3. Soroidea, where they are disposed in an irregular manner 



(Acervulina). 



Lagena is a genus of the Monostega, or single-chambered 

 foraminifers, with a flask-shaped shell, sometimes presenting 

 a beautiful fluted exterior. Entosohnia is like a 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 Nummidiles, 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 palaeozoic 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. Dentalina communis, Orbitolites 

 complanatus, Rosalina italica, and Rotalina globidosa, all living 

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

 licata ranges to the gault ; and Webhina mtgosa 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 every 

 beach ; and they have been obtained in this condition from 

 great depths of the mid-channel. 



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

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



