EECENT RESEARCHES ON THE RA.DIOLARIA. 151 



All the skeletal parts yet treated of are placed without the 

 capsule ; but a circumferential skeleton may exist within, or simul- 

 taneously within and without it. 



Thus we may have a spheroidal shell with small perforations as ■ 

 in Goelodendrum^, and this may be accompanied with one (ITelio- 

 discusf) or two {ActinommaX) other concentric shells external 

 to it. 



The circumferential skeletons hitherto considered have been 

 more or less nearly spheroidal ; but they may be (as in the Dis- 

 cida) so compressed as to form flat, rounded, or elliptical disks, 

 the opposite sides, however, being generally more or less convex, so 

 as to make the whole shell to a greater or less degree lens-shaped. 

 These shells, however, are not merely circumferential, but have a 

 skeletal partition dividing their interior into two by a transverse 

 partition, which extends everywhere to their margins ; and thus 

 they will be better considered when we have entered upon the 

 second or radial system of skeletal parts, to which we may now 

 proceed. 



With the second or radiating system of parts w^e have already 

 unavoidably made some acquaintance in considering the circum- 

 ferential skeleton. Thus in Botryocampe, Eucyrtidium, Eucecry- 

 phalus, Dictyopodium,^, JDictyocJia, Zycjostephanus, and Acanthoder- 

 mia we have seen certain centrifugal spines radiating from the 

 surface of the shell. 



The pure and simple radial system is, however, to be seen at 

 its simplest in PlagiacantJia, where the point of union of the radii 

 does not lie within the capsule but eccentrically beside it. 



Next may be mentioned the long spines of Aulacantha\\, which 

 are hollow with numerous barbs towards their apices, and which 

 impinge by their proximal ends against the outside of the capsule. 

 Also the barbed spines oiAulosph(sra elegantissima*^ ^ which spines 

 radiate from the points of junction of the circumferential hollow 

 bars which form its beautiful and symmetrical investing shell 

 before noticed. 



From the outside of a circumferential shell long spines may 

 radiate in all directions, and from such spines fibres may be given 

 off at regular and coinciding intervals, which fibres may, by their 

 junction, form as many as six delicate concentric spheres, succes- 



* L. c. pi. xiii. fig. 3. t L. c. pi. xrii. figs. 5-7. | L. c. pi. xxiii. fig. 6. 

 § See ante, fig. 6. || ' Eadiolarien,' pi. iv. fig. 2 & 3. ^, L. c. pi. xi. fig. 5. 



