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



rarely circular. Their widest part is generally the distal opening ; more rarely this is 

 a little constricted. The thin transparent lamella of acanthin, representing the mantle 

 of the double cone, is commonly ribbed or furrowed by longitudinal, parallel or. divergent 

 crests, and elegantly denticulated on the edge of the distal opening. 



The two conical or cylindrical halves of the mantle are connected with the two 

 enclosed principal spines not only at the base, where they arise from the small central 

 lattice -shell, but also throughout a certain part of their length, by means of two, four, or 

 six wings or leaves, which lie opposite and in pairs in the meridian planes of those 

 spines. These meridian wings are more or less triangular (with broader concave outer 

 bases), and connected by their axial edge with the spine and by their peripheral edge 

 with the mantle. They separate two, four, or six conical spaces or pyramidal com- 

 * partments in each cone. But these aspinal compartments and the separating septa are 

 not new productions of the Diploconida, but are by inherited from their ancestral 

 family, the Hexalaspida (compare above, p. 873). 



The eighteen smaller spines in Diploconus are either of nearly equal size or more or 

 less differentiated. The eight tropical spines are often much larger than the eight polar 

 spines. The two geotomical spines (or the two opposite equatorial spines of the 

 shortened geotomical axis) are often quite rudimentary. In Diplocolpus the external 

 part (outside the shell) is in all eighteen smaller spines rudimentary or atrophied. 



The Central Capsule, as shown by Hertwig, contains numerous small nuclei, and is 

 divided into three parts by the above named two transverse strictures ; the smaller central 

 part (in the original lenticular lattice-shell) and the two opposite larger parts, filling up 

 the greater part of the two conical or cylindrical sheaths, and more or less adopting their 

 form. Corresponding to the shell itself the enclosed capsule is often more or less flattened, 

 being compressed at both poles of the geotomical axis. The pseudopodia seem to 

 proceed only from the two large polar apertures of the sheaths, and form therefore two 

 opposite conical tufts or bunches. 



Synopsis of the Genera of Diploconida. 



All twenty spines more or less developed (sometimes eight of them rudimentary), 380. Diploconus. 

 Only the two hydrotomical spines developed (all the eighteen others rudimentary), 381. ' Diploc.olpu*. 



Genus 380. Diploconus, 1 Haeckel, 1862, Monogr. d. Eadiol., p. 404. 



Definition. Diploconida with two very large spines (opposite in the 

 ' hydrotomical axis) and ten to eighteen other much smaller spines externally visible. 



Diploconus= Double cone; S/TAoo?, xuvo; 



