128 Guide to the Invertelrata. 



GALLEEY ^^Y ^^ mentioned the different species of Ventriculites, some of 

 X. wliich show the folding or plaiting of their walls very distinctly 

 (Fig. 179). Also the wide flange-like walls of Guettardia, and the 

 peculiar mushroom-shaped specimens of Coeloptychium. In the 

 specimens of this latter from the Upper Chalk of Westphalia, the 

 form and structure are perfectly preserved. The large size 

 and the projecting canal apertures in the Lithistid Stichophyma 

 and Verruculina from the Tlamborough Chalk are also worthy 

 of notice. 



The only representatives of fossil sponges from Tertiary rocks 

 are the borings of species of Cliona in molluscan shells. 



YI.— PROTOZOA. 

 EHIZOPODA. 



GALLEEY 1- — Badiolarta. In this order of the Rhizopoda, the sarcode- 



X- body is differentiated into a central mass of protoplasm of a more 



Table- viscous character, which is inclosed in a porous membrane or 

 case 16. . • i i • 



capsule. The intracapsular sarcode contains vacuoles, nuclei, 



case 9 granules, and fat globules. The exterior layer of sarcode which 

 surrounds the capsule is jelly-like, and from this pseudopodia, in 

 the form of slender radiating filaments, are given off. The 

 majority of Radiolaria secrete a skeleton either of an organic 

 chitin-like substance known as ** acanthin," or of a " silicate of 

 carbon," or of clear glassy silica. Only the skeletons formed of 

 this last-mentioned material are known as fossil. 



There is an extraordinary variety of form in the skeleton of 

 Radiolaria. The simplest type is a spherical shell of open lattice- 

 work (Fig. 180, If), or there may be two or more concentric 

 latticed shells, connected by radial bars which project beyond the 

 shell as long spines (Fig. 180, 1). Latticed ellipsoidal and dis- 

 coidal forms are also common. In another group, the typical 

 shape is an elongated latticed cone or bell, which has frequently 

 a spine at the apex, and others projecting from the basal margins. 

 The cone may be undivided or partially marked off by transverse 

 constrictions (Fig. 180, 3). In all cases the Radiolaria are of 

 microscopic dimensions, and scarcely, if at all, visible to the 

 unaided si^ht. 



