REPORT ON THE RADIOLARIA. 53 



The M n o s p li 33 r i d a comprise all those S p li te r o i d e a iu which the carapace 

 is represented only by one single lattice-shell. Originally this shell is probably every- 

 where an extracajjsular or "cortical shell," which is developed on the outside of the jelly- 

 veil enveloping the central capsule, and serves as a protective carapace for these soft 

 enclosed parts. But with the progress of growth the central capsule becomes larger than 

 the including shell, and sends out through its pores club-shaped prolongations or csecal- 

 sacs (PI. 11, figs. 1, 5 ; PI. 19, figs. 2, 3, 5 ; PL 20, fig. la ; PI. 27, fig. 3). These 

 protruded sacs may fuse together again outside the shell and form a spherical bladder, 

 now enveloping the smaller shell ; the latter now becomes an intracapsular or 

 " medullary shell." 



As P 1 i o s p h 88 r i d a (or Sphceroidea concentrica) we can oppose to the simple 

 Monosphajrida all other S p h se r o i d e a, the lattice-shell of which is composed of 

 two or more concentric shells, connected by radial beams. Probably all Pliosphserida 

 (or at least the greater part of them) arise from the Monosphaerida by centrifugal 

 growth ; two or more radial spines are developed from the surface of the simple 

 lattice-sphere, and are united together by communicating lateral branches, developed at 

 ecpial distances from the centre ; and this same process may be repeated, two, three, 

 four, or more times. In this way originate the characteristic systems of concentric 

 spheres, all united by piercing radial beams which arise from the surface of the inner- 

 most sphere (not from its centre). Eegarding this mode of growth, we can distinguish 

 the innermost as " original " or " primary " shell, and all subsequent ones as " apposed " 

 or "secondary" shells; if the number of concentric shells amount to three or more, 

 commonly both innermost shells lie within the central capsule and are medullary shells, 

 whilst all others lie outside it and are therefore cortical shells. This difference 

 (;an be commonly recognised also in the isolated shell, without its central capsule ; the 

 distance between the cortical and the medullary shells being commonly much larger than 

 the distance between the two medullary shells. 



The D y o s p h ee r i d a, or the 8 p h sb r o i d e a Avith two concentric shells, are the 

 most numerous among the Pliosphaerida. Commonly in this group the inner or primary 

 shell lies within the central capsule as a true "medullary shell," whilst the outer lies outside 

 it as a " cortical shell " ; therefore the radial beams, connecting both, pierce the wall of 

 the capsule. But in several forms, mainly in the peculiar group of Diplosphserida, both con- 

 centric shells remain outside the central capsule, and both are therefore "cortical shells." 

 The T r i s p h 33 r i d a, or the S p h fe r o i d e a with three concentric shells, are also 

 very rich in different forms, though not so numerous by f;ir as the Dyosphterida. 

 Commonly in the Triosphserida both inner shells lie within the central cajasule as 

 " meduUary shells," whilst the third lies outside it as a " cortical shell " ; therefore the 

 central capsule remains intermediate in size between the outer and the middle shell. 

 But iu some genera (ejj., lihodosphcera) both outer shells are cortical and only the inner- 



