REPORT ON THE RADIOLAEIA. 883 



tropical spines, so that around each principal spine the four neighbouring tropical spines 

 (two of the northern and two of the southern hemisphere) had grown together by 

 their edges and formed the peculiar conical sheath. I can now say that this opinion 

 (afterwards adopted also by Hertwig) was quite erroneous, the two conical or funnel- 

 shaped sheaths being the enlarged basal sheaths of the two hydrotomical spines, which 

 we have already seen in the Hexalaspida (PI. 139). But whilst in these latter all six 

 principal spines of the hydrotomical meridian plane are hypertrophied (two equatorial 

 and four polar spines), in the nearly allied Diploconida only the two opposite equatorial 

 spines are developed, whilst all other eighteen spines are more or less atrophied or quite 

 rudimentary. In Diploconus the latter are more or less evident, whilst in Diplocolpus 

 they disappear externally. 



The true lattice-shell of the A c a n t h o p h r a c t a (constantly composed of the 

 meeting apophyses of twenty radial spines) is therefore represented in the Diploconida 

 by the small roundish middle part of the whole shell, which is usually much smaller 

 than the two opposite cones, and separated from them by the two slight transverse 

 strictures. Usually this small but most important middle part of the shell is very 

 dark and opaque, on account of its very thick wall and small pores ; but in some 

 species it is clear enough to ascertain that the structure of this lattice-shell is the 

 same as in the lenticular Hexalaspida, there being a network of thick crests on 

 the outer surface and small pores in the dimples between them. Indeed, in many (and 

 probably in all) Diploconida the forty aspinal pores are present which we found in all 

 Hexalaspida, Belonaspida, and Diporaspida, so that these four families of A c a n t h o- 

 p h r a c t a represent one continuous phylogenetical series ; Phractaspis among the 

 Diporaspida is at the beginning, and Diplocolpus among the Diploconida at the end 

 of this remarkably transformed morphological series. 



The twenty radial spines in all Diploconida are probably united very firmly (or 

 even perfectly grown together) in the centre of the small thick-walled lattice-shell, the 

 inner space of which is extremely reduced. Probably, too, the sutures between the 

 meeting apophyses of the thick radial spines are often (or even constantly) obliterated by 

 concrescence, so that the whole shell forms a single piece of acanthin. But I regret that 

 I cannot ascertain these and other points in the structure of the shell, as the small number 

 of specimens observed did not permit an anatomical examination to be made. I have no 

 doubt, however, that the structure of the whole of the middle main part of the shell is 

 quite the same as in the lenticular shell of the thick-walled Hexalaspida, and that in both 

 families each of the twenty radial spines bears originally only two opposite apophyses. 



The characteristic mantle of the double cone of the Diploconida, or the basal sheath 

 of their two large, perfectly developed principal spines, is usually much larger than 

 the shell itself, and more or less compressed from both poles of the shortened geotomical 

 axis. Therefore the transverse section of the two cones is usually elliptical, more 



