132 PROTOZOA. 



trically within one another, the smaller within the larger, 

 and united by radial bars. Most of the fossil forms, how- 

 ever, belong to the type in which the shells are in the shape 

 of a porous siliceous trellis-work, which may be quite un- 

 divided, or is partially marked off into two or more compart- 

 ments by longitudinal or transverse constrictions. The two 

 poles of the shell are quite unlike one another, and the 

 central membranous capsule of the living animal is enclosed 

 within the closed apical pole. As examples of this type, we 

 may select the genera Podocyrtis, Eucyrtidium, Lychnocanium, 

 and Dictyomitra (see fig. 31), all of which are found in the 

 Tertiary, and the last of which is represented in the Chalk. 

 In another group, of which Dictyocha (fig. 31, d) is the type, 

 the skeleton is composed of irregular bars of flint united into 

 a loose network with wide meshes. This type begins in the 

 Chalk, and is represented in the Tertiary deposits and in 

 recent seas. Lastly, we have a group in which the skeleton 

 consists of a flat or lenticular and biconvex plate, which is 

 sometimes double, and has a more or less complex inter- 

 nal structure. As examples of this group, we may select 

 Astromma, Trematodiscus, Ehopalastrum, Stephanastrum, and 

 Stylodictya, the last of which begins in the Chalk, while the 

 others are represented in the Tertiary. 



* 



III. SPONGIDA. 



The Sponges may be defined as Ehizopoda composed of 

 numerous amazbiform masses of sarcode united into a composite 

 mass, which is traversed by canals opening on the surface, and 

 is almost always supported ly an internal skeleton or framework 

 of horny fibres or of calcareous or siliceous spicula. 



The only portion of the Sponges with which the palaeontol- 

 ogist is concerned, is the skeleton. Whatever the nature of 

 the skeleton may be, it is so arranged that its parts surround 

 two sets of apertures which open on the surface of the sponge, 

 and which are connected with one another by a system of 

 canals ramifying in its deeper portions. Of the apertures 

 which penetrate the substance of the sponge in every direc- 

 tion, one set consists of large chimney-like openings, which 



