1 PALEONTOLOGY. 



Amorphospongia, which have a stony reticulate frame, without 

 spiculse, and are grouped together as Petrospongiadce, passed 

 away with the secondary epoch, and the family has no repre- 

 sentatives in tertiary deposits or existing seas. 



Class II. — Ehizopoda* 



The organisms of this class are small and for the most 

 part of microscopic minuteness, of a simple gelatinous 

 structure, commonly protected by a shell. The most simple 

 rhizopods, called Amoeba, present a globular form when 

 contracted, but can extend portions of their substance (" sar- 

 code") like roots, and use them to draw along the rest 

 of the mass, like the feet or tentacles of polyps, whence 

 the name of the class. These root-like processes can also 

 attach themselves to foreign particles, and draw them into 

 the "sarcode," where the soluble organic part, so "intus-sus- 

 cepted," may be assimilated, the insoluble part being extruded. 

 A solid hyaline corpuscle or nucleus is commonly discernible 

 in the interior of the Amoeba, sometimes accompanied by one 

 or more clear contractile vesicles. When the productions of 

 the sarcode are numerous, filiform, and seemingly constant, 

 radiating from all parts of the body, the rhizopod presents the 

 characters of Actinojphrys. When the tentacles are produced 

 from only one extremity of the body we have the genus 

 Panyphagus. When such a rhizopod is enclosed in a mem- 

 branous sac it is a Difflugia; if the sac be discoid with a slit 

 on the flat surface for the protrusion of the tentacles, it is an 

 Arcella. In other rhizopods the sac is calcified, or becomes a 

 " shell," which is sometimes simple, but usually consists of an 

 aggregate of chambers, inter-communicating by minute aper- 

 tures, whence the name Foraminifera given to the testaceous 

 rhizopods. These chambers grow by successive gemmation 



* Gr. rkiza, root : pons, foot. 



