EEPOET ON THE RADIOLARIA. 457 



plates of the surface are the porous walls of the cortical shell itself, 'ilie correctness of 

 this explanation seems to be proved b)' such forms as figured in PI. 38, figs. 2, 4, 

 where the whole surface of the phacoid shell is covered bj' a concentric chamber-work, as 

 a central continuation of the marginal concentric rings. If we imagine a system of 

 perfect concentric lenticular phacoid shells, compressed strongly from Ijoth poles of the 

 shortened main axis, we get the same figure. 



Rarely one single girdle only is developed on the equatorial margin of the lenticular 

 disk (PL 37, figs. 2, 3, 5). Commonly the number of concentric girdles amounts to 

 three to six, often to ten to twelve or more. Some of these largest Coccodiscida reach a 

 considerable size. Commonly all the girdles are of the same lireadth, which is al)out 

 equal to the radius or to the diameter tif the inner medullar}- shell. Rarely the first (or 

 innermost) girdle differs by its greater breadth from the succeeding ones (PI. 36, fig. 8). 



Only in few Coccodiscida the girdle-building remains restricted to the equatorial 

 planes, so that all the chambers lie in it. Commonly on both sides of this ^Aane 

 become developed several layers, and often the number of these (three to six or 

 more) increases towards the periphery ; in other ('ases not their number, but their 

 height increases. Therefore very often the margin of the discoidal shell is mucli 

 thickened, as thick as the centre of the lenticular phacoid shell (or even more) ; 

 whilst laetween the latter and the former (on the proximal girdles) the disk is 

 considerably thinner (PI. 36, figs. 2, 4 ; PI. 37, figs. 7, 8 ; PI. 38, figs. 2, 4). The 

 stratified layers communicate by large openings between their chambers. The radial beams 

 are commonly more or less regular and piercing, but also frequently irregular and inter- 

 rupted ; often their numl^er increases towards the margin by intercalation of new beams. 



T7ie Pores of the sieve-j^lates, which cover both sides of the chambered disk, appear on 

 the margin of the phacoid shell as direct continuations of the pores of the latter, and some- 

 times they are so regularly disposed that one single circular pore is situated on the 

 surface of each chamber (PI. 36, fig. 7; PI. 37, fig. 1). But commonlj^ the pores are 

 of varialile size and number, two to three on each chamber, and often quite irregulail}' 

 scattered. 



The Margin of the chambered disk exhibits many differences, which afford characters 

 for the distinction of genera. In the first subfamily, the Lithocyclida, the margin is 

 quite simple without radial appendages. In the second subfamily, the Stylocyclida, it 

 is armed with solid radial spines lying in the ec^uatorial plane, and often regularly 

 disposed in the same manner as in the Phacodiscida (compare above, p. 421). In the 

 third subfamily, the Astracturida, the margin bears two or more (commonly three or 

 four) chambered arms, also situated in the plane of the disk, and of the same structure 

 as the circular cham]>ered girdles (PI. 38). In some cases*even the whole system of 

 chamber-girdles is represented only by the radial arms, which are inserted immediately 

 on the margin of the phacoid shell. We may regard therefore these formations as 

 imperfect chambered disks, which are developed only in the direction of certain rays 



(ZOOL. CHALU EXP. P \r;T XL. 188.').) Rf 58 



