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



2. Hymenactura pythagorce, Haeckel. 



Hymeniastrum pythagorce, Ehrenberg, 1854, Mikrogeol., Taf. xxxvi. fig. 31 ; Abhandl. d. k. 

 Akad. d. Wiss. Berlin, 1875, p. 76, Taf. xxx. fig. 5. 



Phacoid shell three time.s as broad as the medullary shell, with six pores on its radius. 

 Arms nearly trapezoidal, somewhat longer than the diameter of the central disk, at the truncated 

 distal end nearly as broad, at the base two-thii'ds as broad. In each arm eight to nine 

 transverse rows of square chambers, each chamber with one pore on the surface. Patagium 

 enveloping the basal half of the arms, with four to five rectilinear parallel rows of chambers, forming 

 together a regular triangle. Differs fi-om the preceding species mainly in the quite different 

 structure of the patagium, from the true Hymeniastrum' ■pythagorce in the phacoid shell of the 

 central disk, which encloses only one simjjle medullary shell. 



Bimendons. — Diameter of the phacoid shell Oil, of the medullary shell 004; length of the 

 arms 013, basal breadth 0'07, distal breadth 012. 



Sahitat. — Fossil in the rocks of Barbados. 



3. Hymenactura trigona, n. sp. 



Astromma sp., Bury, 1862, Polycystins of Barbados, pi. xv. fig. 1. 



Phacoid shell twice as broad as the medullary shell, with four to five pores on its radius. 

 Arms slender, lanceolate, thi-ee times as long as the diameter of the phacoid shell and five times 

 as long as broad in the middle part ; distal ends blunt. Patagium enveloping only the basal 

 half of the arms, forming a regular triangle with concave sides. 



Dimensions. — Diameter of the phacoid shell 0"08, of the medullary shell 0-035 ; length of the 

 arms 2, greatest breadth 0'04. 



Habitat. — South Atlantic, Station 332, depth 2200 fathoms ; fossil in the rocks of Barbados. 



4. Hymenactura hexagona. 



Hymeniastrum sp., Bury, 1862, Polycystins of Barbados, pi. sv. fig. 3. 



Phacoid shell twice as broad as the medullary shell, with four to five pores on its radius. 

 Arms nearly equilateral triangular, three times as long as the diameter of the phacoid shell, at the 

 truncated distal end nearly three-fourths as broad as long, at the narrow base only one-fourth as 

 broad. Patagium complete, enveloping the arms completely, so that the whole body represents 

 a regular hexagonal disk. 



Dimensions. — Diameter of the phacoid shell 0'08, of the medullary shell 0"04 ; length of the 

 arms 018, basal breadth 004, distal breadth 016. 



Habitat. — Fossil in the rocks of Barbados and Nicobar. 



