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



their outer edge (fig. 6, i,g). There are no proper upper arm plates, but (as in Gorgono- 

 cephcdus, Astroschema, and Ophiomyxa) the vault of the arm is strengthened and partly 

 covered by grains, or small plates, lying under the skin. Near the base of the arm they are 

 arranged in a double vertical row (fig. 4, 5,j) ending on the upper median line in a stout 

 piece (/), the base, on which is mounted the peculiar spine of this species (z). The vertical 

 exterior double rows of hook-bearing grains, found in Astrophyton, Gorgonoccphalus, 

 Astroclon, Astrocnida, Astroporpa, Astrogomp>hus, Astrochele, and Astrotoma do not exist 

 at all in this genus, which agrees in this respect with Trichaster, Astroceras, Astroschema, 

 Ophiocreas, and Astronyx. The chief bracing pieces, namely, radial shield and genital plate, 

 are stout and firmly hinged, and the former (fig. 3, /), instead of being composed of united 

 overlapping scales, is solid ; while the latter (o) is firmly bedded in a series of soldered 

 plates, which connect it with the side arm plates (i). Attached to the articulation is a 

 short, very stout, genital scale («.). The arm bones are lower and wider than among 

 kindred genera, but are jointed in the usual way ; that is to say, the inner face presents 

 a vertical prominence constricted in the middle (fig. 14, /3), while the outer face has a 

 similar but horizontal prominence (fig. 13, y), and the two, held together by muscles and 

 skin, make a free-playing joint. At each forking of the arm a curious modification takes 

 place. The bone, while retaining its general form, is much widened and is split vertically 

 almost in two (fig. 16) ; on the inner face of each half is a vertical hour-glass prominence 

 (j3), and the outer face of the arm bone next within is suitably modified (fig. 15) by being 

 much widened, and by having, at its constricted part, an articulating peg, or wedge, 

 which fits into the hollow between the two vertical hour-glasses just described. At the 

 joint outside these, the forking is perfect, and each prong has an arm bone of nearly the 

 normal shape. 



Table of Species of Astrophyton. 



Disk with very high radial shields hearing a few stout stumps, covered ) . . , . 



with thick skin, and often fluted, . . . . . f Astrophyton costosum. 



° .3 "? 

 '5 "tc--5 

 2 a -^ 



5^ ~ i» 

 t-i t-i £ 



r- ~ H 



S.S -2 



■2 .a 



OS 



<D ^ " 

 i-- "^ ^ 



^-^ **-! Ph 

 ^ » C « 



8 <s g 



-3 <d ° .s 



- CD a 



PR 



No tentacle scales on pores. Disk and arms quite smooth, the latter ) . . , . , 



with faint belts of hooklets, . . . . { f Astrophyton nudum. 



Radial shields closely beset with small thorny stumps, . . . Astrophyton clavatum. 



Upper disk and arms set with smooth grains of several sizes, . . Astrophyton exiguum. 



Disk set sparsely with minute, short, slender spines, which are continued ) . , , . 



in groups of three along upper surface of arm, . . ) A ^ophyton spmosum. 



Disk covered with fine, close-set grains, which form cross ridges on the ] 



radial shields ; and on the arms, belts alternating with those of the \- Astrophyton eoscilia. 

 hooklets, . . . . . . . . ) 



Disk and upper surface of arm set with spaced grains, which are fine and ) . . , . 



nearly equal, . . . . . ] Astrophyton panamense. 



