1012 THE VOYAGE OF H.M.S. CHALLENGER. 



3. Lithocubus vinculatus, Haeckel. 



Acanfhodesmia vinaulata, J. Miiller, 1856 (jiartim), Abhandl. d. k. Akad. d. Wiss. Berlin, 

 Taf. i. fig. 7 (not 4-6). 



The twelve rods of the cubical shell are curved and armed with scattered, simple, short spines. 

 The figure 7 of Johannes Miiller (loc. cit.) corresponds exactly to the Mediterranean form observed 

 by me at Portofino, and is quite different from his true Acanthodesmia vinculata (loc. cit., figs. 46), 

 so that I have no doubt he did observe these two different species (compare above, p. 975). 



Dimensions. Diameter of the cube 0'07 ; length of the spines 0'02. 



Habitat. Mediterranean (Nice, Portofiuo). 



4. Lithocubus astragalus, n. sp. (PI. 82, fig. 12). 



The twelve rods of the cubical shell are stout and slightly curved, armed with numerous, 

 simple, and irregularly branched spines. Eight larger spines, branched like a deer's antler, arise 

 from the eight corners of the cube. 



Dimensions. Diameter of the cube -O'l to 0'12 ; length of the corner spines 0'05 to 0'08. 



Habitat. Central Pacific, Station 271, depth 2425 fathoms. 



Genus 439. Circotympanum, 1 n. gen. 



Definition. Tympanida with two simple different horizontal rings, connected by 

 numerous (six to eight or more) divergent columellse. 



The genus Circotympanum comprises those Tympanida in which two simple hori- 

 zontal rings of different size (an upper smaller " mitral ring," and a lower larger " basal 

 ring ") are connected by a variable number of divergent rods or columellaa (at least six or 

 eight). Circotympanum is probably derived from Tympaniscus or Tympanidium by 

 partial reduction of the sagittal ring, the upper (apical) and lower (basal) part of which 

 is lost. The geometrical fundamental form is a truncated pyramid with six to eight or 

 more edges. 



1. Circotympanum hexagonium, n. sp. 



Mitral and basal ring hexagonal, the latter twice as broad as the former, each with a simple 

 small spine on the six corners. Six columellse nearly straight, as long as the radius of the basal 

 ring. All rods with three prominent edges. 



Dimensions. Breadth of the mitral ring 0'06, of the basal ring O'll. 



Habitat. Central Pacific, Station 268, depth 2900 fathoms. 

 1 Circotympanum = Drum with rings ; x/jxoj, 



