ASTEEOIDEA OF H.M.S. * CHALLEN&ER ' EXPEDITTO^^ 207 



Caltpteastek, 11. gen. 



Form depressed, marginal contour pentagonal, dorsal area plane, actinal 

 area convex. Supradorsal membrane verj' delicate, perfectly transparent. 

 No muscular fibrous bands. Spiracula present. No spicules. Paxillse 

 with short robust spinelets (6 or 6 in the type species), usually flaring 

 at the extremity, crown widely expanded, not protruding through, or 

 even elevating, the membrane, simply supporting it. Ambulacral spines 

 forming ti'ansverse combs ; spinelets perpendicular in position, webbed 

 together. Segmental apertures present. Aperture-papillaj not free, open- 

 ing laterally only. Mouth-plates with two or three pairs of superficial 

 secondary spines. Mouth-spines proper 2, or occasionally 3. Actino- 

 lateral spines merged in the actinal floor. No independent lateral fringe. 



This genus is established for the reception of a very elegant 

 little form which presents certain resemblances to Hymenaster 

 and certain to Pteraster. It resembles JSymenaster in the ab- 

 sence of a marginal fringe, and in the merging of the actino- 

 lateral spines in the actinal floor, and resembles Pteraster in its 

 transverse combs of ambulacral spines united by web-tissue. It 

 differs from both genera in its transparent supradorsal membrane 

 devoid of muscular fibrous bands, by the possession of three pairs 

 of free secondary superficial mouth-spines, and by its true pen- 

 tagonal form. 



Caltpteastee coa, n. sp. 



Marginal contour pentagonal, interradial angles very slightly 

 incurved. Minor radial proportion 68 per cent. ; -S=ll millim., 

 o'=7'5 millim. Dorsal surface flat, or even somewhat concave in 

 consequence of the rays being slightly curved upward; radial 

 areas not specialized externally, the dorsal membrane forming a 

 plane area. No marginal fringe. Actinal surface convex. 



Supradorsal membrane extremely thin and perfectly hyaline, 

 a clean specimen appearing to the eye as transparent as glass. 

 No muscular bands present, although a flbrillar structure may be 

 seen in the tissue under high magnification. Spiracula rather 

 large, numerous, and uniformly distributed. Paxillfe with long 

 pedicels, moderately delicate, evenly and equidistantly placed, 

 usually with five spinelets, which are short in comparison with 

 the pedicel, and rather delicate. A few of the spinelets taper 

 a little at their extremity ; but usually they are slightly flaring. 

 The supradorsal membrane is literally supported upon the tips 

 of these spinelets. About nine lougitudinal rows of paxillae- 

 crowns are discernible at the base of a ray. The oscular 



