170 



PEOF. ST. GEOEGE MIVAET ON 



Multipolar growth is also exemplified in such forms as Dora- 

 taspis, Haliommatidiim, Bhizosph(Bra, Diplosphcera, and Actino- 

 sphcera. Such shells arise by extension of the skeleton from 

 the radii outwards at right angles, and at similar distances from 

 the centre of the shell, the lateral extensions from such radii 

 meeting to form a sphere by their junctions. In the same way, by 

 the development from eacli radius of successively diverging struc- 

 tures at similar distances are built up many of the successive 

 concentric spheres before described, i. e. of those which coexist 

 with radii. 



In Coelodendrum alone of all Eadiolaria is constant absorption 

 and redeposition known to take place. It must do so here, as 

 the radiating tubules have always thin w\alls. 



Unipolar growth is exemplified in the Poly cystine forms, in which 

 at first the shell is a mere cap placed on the apex of the capsule, 

 and thence growing forth on all sides — in the same way the 

 chambers of such more complex Polycystine forms as Eucyrtidium, 

 and probably also those with closed bases like Botryocampe, though 

 such have not, so far as I know, yet been discovered in process 

 of formation. 



The hilaieral mode of growth is exemplified by Haliodiscus * , 

 in which the outer shell arises as two shields, one on each side of 

 the central shell, each enlarging at its circumference till the two 

 meet. 



The Discida are formed by a process which may be considered 

 a combination of the bilateral and the multipolar modes. Their 

 median partition is formed by radii which, by the processes they 

 give off" at right angles, form the series of concentric or spirally 

 arranged chambers which exist in the median partition and be- 

 tween it and the two outer shields. The two outer perforated 

 plates or shields are formed in the same way as are the bilateral 

 plates of Haliodiscus, being, however, nearly quite parallel instead 

 of being much curved as in the last-named genus. 



The shells with a spongy skeleton are formed in one or other 

 of the before mentioned ways — in Spongurus certainly in the mul- 

 tipolar way. 



Distribution oftJie Eadiolaria. 

 The Madiolaria have as yet been found in salt water only, but 

 there yqyj abundantly close to the surface of the sea and at a little 

 depth beneath the surface, whence they descend in cool or cloudy 

 weather. Until quite recently there was no evidence that they 

 * Muller, Abhand. d. k. Akad. Berlin, 1858, pi. ii. figs. 5 & 6. 



