454 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



Family MYXASTERIDAE Perrier. 



Genus MYXASTER Perrier. 



Myxaster Perrier, Comptes Rendus, vol. 101, 1885, p. 886. Type, M. sol 

 Perrier. 



MYXASTER MEDUSA (Fisher). 



Plate 134, figs. 2, 2ar-c. 

 Asthenactis medusa Fisher, 1913c, p. 224. 



Diagnosis. — Rays 9. R=225 mm., r=52 mm., R=4.3 r; breadth 

 of ray at base, 25 mm. Rays very flexible, slightly swollen at base, 

 tapering, elevated, and somewhat trigonal in section; integument 

 yielding, the plates very thin, and the skin soft and slimy ; spinelets 

 about 6 to a fascicle, the fascicles membrane invested and close-set; 

 adambulacral spines webbed, 9 or 10, the outermost belonging to the 

 next, adoral, plate; mouth spines, 13 or 14, webbed. Differing from 

 Myxaster sol in having more numerous and longer mouth spines, a 

 thicker skin, and 2 instead of 3 spines on the apophysis of each adam- 

 bulacral plate. 



Description. — Abactinal plates very thin, imbricated, each bearing 

 a fascicle of extremely delicate spines, usually about 6, although 

 sometimes slightly more numerous, and resembling fine glassy hairs. 

 The abactinal surface is covered with soft membrane, which forms an 

 investment to the spines. The fascicles are 6 to 8 mm. long and 

 spaced 2 or 3 mm. apart. The form of the plates is variable; some 

 are four-lobed, others hourglass shaped. The figures will give a 

 better idea of the form. Owing to the softness of the integument and 

 the overlying slime I have not been able to discover any papulae on 

 the outside, but an examination of the coelomic surface of a portion 

 of the dorsal body wall shows single papulae in the interspaces be- 

 tween the lobes of the plates. 



Adambulacral armature similar to that of Asthenactis papyraceus 

 (but lacking, of course, the actinolateral membrane). Each curved 

 oblique comb consists of 9 or 10 very delicate spines, webbed to their 

 tips and increasing in length from the inner to tlio outermost, 

 the last spine standing on an extension of the next adorally situated 

 plate. The innermost spine is one-fourth to one-third the length 

 of the outermost, and the latter is joined to the side of the ray 

 by a flange-like extension of the web, about as broad at base as one- 

 third the length of outer spine. This flange of tissue narrows rapidly 

 in width up to the tip of the spine, and near the outer edge envelops 

 a tiny fascicle of 2 or 3 setalike spinelets of a lateral plate. These 

 are hidden by the membrane. 



Mouth plates large, with a straight median suture, the form and 

 armature being shown by the figure. The marginal spines form 



