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



shell. Eighteen other spines (disposed according to the MuUeriau law of Icosacantha) 

 much smaller, often rudimentary. Central capsule ellipsoidal or diploconical. 



The family D ij^l o c on i d a, founded by me in 1862 for a single Mediterranean 

 species {Dijyloconus fasces), appears to be the most aberrant and strange form among 

 the AcANTHARiA. As I had met with only a single specimen, very dark and intrans- 

 parent in its central part, my observations on its structure were imperfect and the 

 explanation of it partly erroneous (compare my Monograph, pp. 46, 404, Taf. xx. 

 figs. 7, 8). However, I regarded Diploconus as the representative of a quite peculiar 

 family, derived from the Acanthometrida, and I correctly compared the large opposite 

 radial spines of one equatorial axis wdth the corresponding parts in Amphilonche. 



Afterwards Richard Hertwig observed some specimens of Diploconus fasces in the 

 same locality (Messina), and gave an accurate description of its central capsule, 

 including numerous small nuclei (1879, Organismus d. Radiol., p. 28, Taf ii. fig. 3). 

 He found also that the peculiar diploconical skeleton is not composed of silex, but of 

 acanthin. In the explanation of the shell-structure he adopted my opinion. 



In the rich collections of the Challenger I detected ten diff"erent forms of Dijalo- 

 conida, all very rare, and for the most part represented only by single specimens. A 

 twelfth species was found by me in the collection of Captain Rabbe from the Indian 

 Ocean. By the study of these new forms, and particularly by their comparison with the 

 most nearly allied Hexalaspida and Belonaspida, it was possible for me to correct some 

 errors in my former description and to give a much more correct description and natural 

 explanation of this very peculiar and strange family of Radiolaria (compare PI. 140). 



The most characteristic and the most voluminous part of the acanthinic skeleton 

 in all Diploconida appears as the diploconical or nearly cylindrical solid " mantle " 

 giving them their name and odd appearance (PI. 140). Usually this mantle is 

 broader on its two opposite terminal openings than in its more or less constricted 

 middle part. This latter is now more spherical or ellipsoidal, now more lenticular, and 

 usually separated from the two cones by two slight transverse strictures. On the 

 surface of this middle part twelve to eighteen radial spines, which in Diplocolpus are 

 rudimentary or absent, are visible in Diploconus. The longitudinal axis of this shell 

 is constantly occupied by a very large pair of opposite stout prismatic or cylindrical 

 principal spines, which are united in the centre and usually more or less prominent 

 with their distal apex over the two openings of the double cone. 



In my first communication on Diplocoivus (1862, loc cit.) I correctly comjaared 

 these two large spines in the prolonged main axis of the shell to the princijjal 

 equatorial spines of Amphilonche (or to the " hydrotomical spines," c\, c3) ; but 

 my explanation of the two peculiar cones enveloping them was erroneous. I supposed 

 at that time that they were formed by the eight flattened and leaf-shaped curved 



