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



their outer edge (fig. 6, ?,(/). There are no proper upper arm pLates, but (as in Gorgono- 

 cephalus, Astroschema, and Ophionvjxa) the vault of the arm is strengthened and partly- 

 covered by grains, or small 2:)lates, 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 (2). The vertical 

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

 Astrodon, Astrocnida, Astropoipa, Astrogomphus, Astrochele, and Astrotoma do not exist 

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

 OpJiiocreas, and Astromjx. The chief bracing pieces, namely, radial shield and genital plate, 

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

 overlapping scales, is sohd ; while the latter (0) is firmly liedded in a series of soldered 

 plates, which connect it with the side arm plates (/). 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, fS), 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 

 |)laee. 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 

 (/8), 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 Ijone of nearly the 

 normal shape. 



Table of Species of Astrophyton. 



Disk with very lii"li radial sliielils bearing a few stout stump.'?, covered ) , , , , , 



.,,,,■ V ,? 1 i-i jj i. 1 } Astropnmoncomosnm. 



with thick skill, and otten iluted, . . . . . ) ' ■^ 



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



•i-i, i • i. 1 !■. i^ 1 1 1 i t Astrophinon nur/nni. 



with taint baits ot hooklets, . . . . . I -' ' 



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



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



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



^ "V ., , r n^ ' } Astrophiitonspniosum. 



m groups 01 three along upper surface of arm, . . .J t j ±- 



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 ccecilia. 

 hooklets, . . . . . . . . ) 



Disk and upper surface of arm set witli sisaced grains, which are fine and 1 , , , , 



nearly equal, . . . . . /j Astrophyton pananiense. 



^ t-i '^ 



a oj 



.1 i 



o 2 



"7^ Cd C X 



o cs g 



rg <B O g 



TO P ^ 



m .2 -^ S. 



;^ TO fl .^ 



