796 THE VOYAGE OF H.M.S. CHALLENGER. 



Each small plate or aglet is pierced in its centre by a single radial canalicule or 

 porule. The dimpled surface, so produced, resembles somewhat the dimpled plates of 

 Ceriasins, &c. Different from these innumerable very small dimples of the surface are 

 the twenty larger " spinal dimples," or the concave larger plates, which are originally 

 pierced by the twenty radial spines. Before we describe these, we must examine the 

 spines themselves. 



The twenty radial spines of all observed Sphserocapsida (sixteen species) agree 

 perfectly with those of the genus Acanthoma (p. 749), and especially with Acanthoma 

 tetracopa, Acanthoma denticulata, &c. All twenty spines, regularly disposed 

 according to- the Mlillerian law of the Icosacantha, are of equal size, constantly four- 

 edged prismatic, of equal breadth throughout their whole length. The prominent 

 four edges are parallel, sometimes smooth, at other times elegantly denticulated. The 

 central bases of the twenty spines are pyramidal, without leaf-cross, and propped one 

 upon another with their triangular faces, as in the majority of the Acanthonida. 



The relation of the twentj^ radial spines to the spherical shell exhibits in the five 

 genera described very peculiar and important differences. In the first described genus, 

 in SphcBTOcapsa, the spines are exactly as long as the shell-radius, and therefore are not 

 prominent over the surface of the shell, with which they are firmly connected ; the 

 truncated distal end of the spine lies therefore here in the surface of the shell itself, and is 

 connected with it by its four edges, between which four open aspinal pores remain, as in 

 Tessaraspis, &c. (PL 135, figs. 6-10). In the next allied genus, Astrocapsa (PI. 133^ 

 figs. 9, 10), the spines are longer than the shell-radius, and therefore more or less pro- 

 minent over its surface ; the piercing part of each spine is also surrounded by four 

 aspinal pores. In the two following genera, Porocapsa and Cannocapsa (PI. 133, 

 figs. 7, 8), the radial spines are shorter than the shell-radius and therefore quite 

 hidden and withdrawn inside the shell, which they do not reach. But in the ideal pro- 

 longation of each spine the shell is pierced by a single large opening, the " perspinal 

 pore" or "perspinal hole," composed of the four united aspinal pores. Whilst in 

 Porocapsa the perspinal pores are simple, they are jjrolonged in Cannocapsa into 

 cylindrical tubes, ojDen at both ends. The twenty perspinal holes of these Porocapsida, 

 are therefore derived by confluence of the eighty original aspinal pores of the Astro- 

 capsida and preserve the same regular disposition, according to the Miilleriau law of 

 the Icosacantha. Finally, the same law as is valid also in the last genus is found in 

 Cenocapsa; here the radial spines have completely disappeared, and the whole skeleton 

 is a simple sphere, but of the same structure, and with the same twenty perspinal pores 

 as in PorocapscL It is very interesting that this spineless Cenocapsa among the 

 AcANTHAKiA exhibits the same shell (a simple hollow sphere) as a last reduced form,, 

 which Cenosphcera among the Sphaerellaria produces as a primitive ancestral form, 

 of numerous genera. 



