298 BULLETIN 100, UNITED STATES NATIONAI. MUSEUM. 



Genus LITHOSOMA Fisher. 



Lithosoma Fishee, 1911c, p. 422. Type, L. actinometra Fisher. 



Related to Iconaster Sladen, but differs in having the regular longi- 

 tudinal radial series of abactinal plates completely surrounded by 

 granules, not on the lateral edges only, and in having the subam- 

 bulacral granulation short and spaced from the furrow comb, not 

 crowded and graduated into the furrow armature. All plates smooth 

 and bordered by a single series of granules, the plates of papular 

 areas sensibly elevated. Small spatulate excavate pedicellariae on 

 both surfaces. Disk large, rays long, slender, and beyond base of 

 ray composed only of marginal plates abactinally. 



KEY TO THE SPECIES OF LITHOSOMA HEREIN DESCRIBED. 



c* R=3.7 r ; marginal plates broad, more or less tumid ; all superomarginals 

 broader than long; width of ray at proximal end of first pair of supero- 

 marginals which meet medially equal to length of first 5 superomarginals ; 

 furrow margin of adambulacral plates slightly convex, slightly angular, or 

 concavo-convex, but not markedly angular ; adambulacral pedicellaria very 

 small, the jaws much less than half as high as furrow spinelets. 



actinometra, p. 298. 



a.* R=4+r; marginal plates narrow, not at all tumid; distal superomarginals 

 longer than broad ; width of ray, measured as in o,* equal to length of first 

 I] to 3J superomarginals ; furrow margin with an apophysis separating the 

 tube feet; adambulacral pedicellaria with jaws nearly or quite as long 

 as the furrow spinelets penichra, p. 301. 



LITHOSOMA ACTINOMETRA Fisher. 



Plate 76, figs. 1, 3 ; plate 84, fig. 2 ; plate 85, fig. 2 ; plate 93, figs. 3, 3a. 



Lithosoma actinometra Fishes, 1911c, p. 422. 

 Diagnosis. — Rays 5. R=155 mm., r=42 mm., R=3.7 r; breadth of 

 ray at base, 49 mm., at eighth superomarginal, 19 mm. General 

 form precisely like Nymphaster; disk large, arcuately pentagonal, 

 and produced at the corners into long slender rays which gradually 

 taper to a bluntly pointed extremity and which are composed abac- 

 tinally of the marginal plates only. All plates smooth, porcelain- 

 like, bordered by a single complete series of granules flush with the 

 general surface; only the adambulacral and mouth plates with gran- 

 ules on surface. Abactinal plates regular, hexagonal, slightly ele- 

 vated, close-set; superomarginal plates massive, wider than long, the 

 seventh to ninth meeting medially, and encroaching conspicuously 

 upon abactinal area; inferomarginals similar to superomarginals; 

 actinal intermediate plates numerous, extending slightly beyond 

 middle of R ; adambulacral plates slightly wider than long to slightly 

 longer than wide, with a curved or slightly angular furrow margin 

 bearing a comb of proximally G to 8, and farther along the ray 9 to 

 11 short blunt spinelets. 



