794 



THE VOYAGE OF H.M.S. CHALLENGER. 



As the fourteen smaller spines develop their apophyses at smaller distances from the 

 centre, the shell assumes a peculiar lenticular or discoidal form, and the margin of 

 this disk bears the six larger spines. Moreover the enclosed small central capsule is 

 lenticular. The Hexalaspida may be derived immediately from the Belonaspida. 



The Diploconida (PI. 140) form the last and the most modified family of 

 all Acanthophracta. The remarkable shell exhibits the strange form of a 

 double cone, bearing in its axis two very large opposite spines ; these are the two 

 equatorial spines of the " hydrotomical axis" (p. 719). The double-conical or nearly 

 cylindrical shell is composed of three different parts or segments ; the small middle part 

 is the true lattice-shell of the Hexalaspida and Belonaspida, and bears the eighteen 

 smaller (often quite rudimentary) radial spines. The two other parts (opposite on both 

 poles of its hydrotomical axis) are the conical or cylindrical, solid, basal sheaths of 

 the two large equatorial spines, enveloping their major part. In consequence of 

 this peculiar metamorphosis of the shell the Diploconida represent the last and the 

 most aberrant group of all ACANTHARIA. 



Synopsis of the Suborders and Families of Acanthophracta. 



Shell spherical, simple, pierced by twenty or 

 eighty aspinal pores and composed of a 

 pavement of innumerable very small plates 

 or aglets, each pierced by one porule, 



Suborder I. SPH^EROPHRACTA. 

 Twenty radial spines of equal size. 

 Shell spherical (or an endospherical 

 polyhedron). 



Suborder II. PRUNOPHRACTA. 

 Twenty radial spines of unequal size ; 

 two or six hydrotomical spines much 

 larger than the eighteen or fourteen 

 others. Shell not spherical. 



Shell spherical, simple, composed of the 

 meeting branches of two or four apophyses 

 of the twenty radial spines, 



Shell spherical, double, composed of two 

 concentric lattice-spheres, which are con- 

 nected by the twenty radial spines and 

 composed of the meeting branches of their 

 apophyses, .... 



Shell ellipsoidal, with prolonged hydro- 

 tomical axis, the two spines of which are 

 larger than the eighteen others, . 



1. SPH^EROCAPSIDA. 



2. DORATASPIDA. 



3. PHRACTOPELTIDA. 



4. BELONASPIDA. 



Shell lenticular or discoidal, with six larger 

 spines placed in the hydrotomical plane 

 (fourteen other spines much smaller), . 5. HEXALASPIDA. 



Shell diploconical or nearly cylindrical, with 

 two opposite large funnels, the sheaths of 

 the enlarged two spines of the hydro- 

 tomical axis (eighteen other spines much 

 smaller or rudimentary), . . 6. DIPLOCONIDA. 



