REPORT ON THE RADIOLARIA. 405 



The Porodiscida (Pis. 41-47), the largest family of all D i s c o i d e a, begins the 

 series of C y c 1 o d i s c a r i a, or those Discoideain which there is no " phacoid shell," 

 but a small simple central chamber surrounded by a number of small latticed chambers of 

 nearly the same size and form. In the Porodiscida these chambers are arranged in com- 

 plete circular concentric rings or spiral convolutions ; in the small family of Pylodiscida 

 the central chamber is surrounded by three radial arm-chambers separated by three open 

 spaces ; in the third family of Cyclodiscaria, the Spongodiscida, all the chambers 

 are arranged more or less irregularly, and the whole disk becomes spongy ; also the surface 

 of the disk is spongy, whilst in both former families it is covered by two regular 

 even porous plates or "sieve-plates." Probably all Cyclodiscaria can be derived 

 phylogenetically from Archidiscus, a very small and simple lenticular disk, which is com- 

 posed of a small spherical latticed' central chamber and of a single concentric chambered 

 ring or girdle ; the margin of this ring is connected with the central chamber by a 

 variable number of radial beams. This Archidiscus can be derived either from Satur- 

 nalis (PL 13, fig. 6) by the development of lattice- work between the equatorial ring and 

 both polar faces of the concentric central chamber, or from Sethodiscus (PL 33, 

 figs. 1-3) by flattening of the lenticular shell, so that the enclosed inner medullary shell 

 (the central chamber) meets the outer phacoid shell at both poles. 



The Porodiscida are commonly flat or biconvex (rarely biconcave) disks, the central 

 chamber of which is surrounded not by a single, but by a variable number (commonly 

 three to six) of concentric chambered girdles or rings ; they arise from Archidiscus by 

 apposition of new concentric chambered rings around the first ring, all lying in the 

 equatorial plane. Afterwards the disk often becomes thickened by apposition of concen- 

 tric chamber-rings on both flat sides also, so that two to four or more layers are stratified 

 one over the other. The circular concentric rings often become interrupted, or spirally 

 convoluted (wholly or partially) ; also the chambers sometimes become irregularly 

 crowded. But in all cases both surfaces of the disk (upper and lower) continue to be 

 porous plates or sieve-plates, at least in the centre, but they never become spongy. 



The margin of the disk exhibits in the Porodiscida a great variety of different forms, 

 serving for distinction of subfamilies and of genera. In the Trematodiscida the margin 

 remains quite simple, as in the Archidiscida, or is only surrounded by a hyaline equa- 

 torial girdle. In the Ommatodiscida it is distinguished by one or two peculiar oscula, 

 surrounded by a corona of spines. The Stylodictyida are distinguished by a number of 

 solid radial spines, and the Euchitonida by a number of chambered, or spongy, radial 

 arms, arising from the margin of the disk and lying in the equatorial plane. The 

 variety of these radial marginal appendages is in the Porodiscida much greater than in 

 the Coccodiscida. 



The Pylodiscida (PL 48, figs. 12-20) represent a new, small, but very remark- 

 able family of D i s c o i d e a, all triradial, and distinguished by the peculiar formation of 



