REPORT ON THE OPHIUROIDEA. 245 



shallow median canal. Their outer surface lias a large, round mamelon (an exasperation 

 of the articulating shoulder) having above a central gulley to admit the umbo of the 

 next bone ; and below a minute articulating peg with a small depression on either side to 

 admit the knobs of the succeeding bone. The whole is not unlike Ophiochondrus 

 (PI. XLIII. figs. 12, 13), and shows a leaning towards the Astrophytidee. By doing 

 away on the outer face with the articulating peg, and deepening the central gulley, we 

 get a shape like Sigsbeia, and by flattening the tentacle sockets in this last we arrive at 

 the proper transverse hour-glass projection. 



Taking then the inner face, and widening the umbo above (fig. 13, 1) and the knobs 

 below (2), there results a form like that of Sigsbeia (fig. 6), and by further enlarging this 

 figure and diminishing the muscle fields, there is produced a true vertical hour-glass 

 projection (compare PI. XXV. figs. 24, 25). In a word, the Astrophytons, with their 

 slender arms, rolling in a vertical plane, and furnished with small tentacles, have simple 

 hour-glass joints, muscle fields little and flat, and small tentacle sockets. 



The short mouth angle of Ophiomyxa is covered quite to the inner edge of the nerve 

 ring by a pair of stout, triangular peristomial plates. A further Astrop>hyton-like feature 

 is the arrangement of the arm plates, whereof the under one is small (PI. XLIII. fig. 2), 

 and sometimes even in three pieces (Ophiomyxa pentagona), and has attached to it a 

 massive side arm plate (figs. 1 and 3, i), of which the greater portion is on the under side 

 of the arm, bearing stumpy, thorny spines (jj), and continued upward by a number of 

 thin pieces homologous with upper arm plates (j). In Ophiomyxa vivipara, however, 

 there are no upper arm plates, a want which brings the species near Op)liioscolex. 



See Plate XLIII. fip-s. 1-3. 



Table of Species of Ophiomyxa. 



Although Ophiomyxa has only four species, a table of them is given, because they 



are distinguished chiefly by internal characters. 



Radial shields short and thick. Marginal disk scales small and thin. Under arm i 



plates in three pieces. Upper arm plates in two pieces. Disk skin with small r Ophiomyxa pentagona. 

 thin lime plates, . . . . . . . . ' 



Radial shields long and rounded. Marginal disk scales large, thick, few. Under \ 



arm plates wider than long with an outer notch. Upper arm plates in several K Ophiomyxa flaccida. 

 pieces on each side. Disk skin thick, with small scattered lime nodules, . I 



Radial shields short and wide. Marginal disk scales small, numerous, regular. \ 



Under arm plates as Ions as wide with an outer notch. Upper arm plates in ' _ , . , , . 



, , , ., p , , . t • i 1- i Ophiomyxa australis. 



several pieces, and connected along ridge ot arm by other pieces. Disk skin I 



thin and naked, 



Radial shields short, wide, flat. Marginal disk scales small and narrow. JN T o upper j 



arm plates. Under arm plates hexagonal with re-entering curves. Disk skin V Ophiomyxa vivipara. 

 beset with calcareous plates, 



(ZOOL. CHALL. EXP. — PART XIV. 1882.) 32 



