REPORT ON THE OPHIUROIDEA. 245 



shallow median canal. Their outer surface has a largo, round mamelon (an exaggeration 

 of the articulating shoulder) having above a central guUey 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 Opliiocliondms 

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

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

 get a shape like Sigsheia, 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 Sigsheia (fig. 6), and by further enlarging this 

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

 projection (compare PL 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 Astrophyton-\i\ie feature 

 is the arrangement of the arm jjlates, whereof the under one is small (PL XLIIL fig. 2), 

 and sometimes even in three pieces {Ophiomyxa p>e7itagona), and has attached to it a 

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

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

 thin pieces homologous with upper arm plates (y). In Ophiomyxa vmpora, however, 

 there are no upper arm plates, a want which brings the species near Ophioscolex. 



See Plate XLIIL figs. 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 | 



plates in three pieces. Upper arm plates in two pieces. Disk skin with small r Ophiomijxa p(>ntaijona. 

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



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



arm plates wider than long with an outer notch. Uj)per arm plates in several \ Opliiomijxu flaccid a. 

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



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



Under arm plates as long as wide with an outer notch. Upper arm plates in v ,, , . , , . 



,. , 11 -1 ,. 1 1- 1 • 1 1 • (' Opluomijxa OMstralis. 



several pieces, and connected along ridge ot arm by other pieces, i'jsk skm I 



thin and naked. 



Radial shields short, wide, flat. ]\Iarginal disk scales small and narrow. Ko upper \ 



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

 teset with calcareous plates, . . . . . . . ■' 



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



