278 BULLETIN OF THE 



OPHIOBRACHION gen. nov* 



Entire animal clothed in a thick skin which hides the underlying plates and 

 is beset on the disk with spines. Arms long, slender and serpentine. Upper 

 arm-plates wanting. Side arm-plates forming a low ridge with a line of little 

 mamelons each bearing a hooklet which answers to an arm-spine. Tentacles 

 large and simple. Mouth-angles rounded, covered with thick skin and bear- 

 ing at their apex a clump of spiniform papillae. Two large genital openings in 

 each interbrachial space. 



The presence of hooklets, which replace the arm-spines quite to the base of 

 the arm, is a feature found in no other Ophiuran; and, indeed, in none of the 

 Astrophytons; for the hooklets of these, when found at the base of the arm, 

 are not homologous with arm-spines, because they do not stand on the side 

 arm-plate. At the tip of the Astrophyton arm there are, however, strictly 

 homologous spines, like compound hooks, but these, as they approach the base 

 of the arm, change gradually to short thick spines, or tentacle-scales, as they 

 may also be called. (See Bull. M. C. Z., VI. 2, Plate XIX. Figs. 493-495.) 

 As might be expected, the joints between the arm-bones are on the modified 

 hour-glass pattern, similar to that found in Sigsbeia, and therefore approach- 

 ing the corresponding structure among Astrophytons. In a word, this new 

 genus belongs with such genera as Ophiomyxa and Ophiobyrsa, and stands 

 nearest the simple-armed Astrophytons. 



Ophiobrachion uncinatus sp. nov. 



Plate VIII. Figs. 128-131. 



Special Maries. — Arms six or seven times the diameter of disk. Seven or 

 eight compound hooklets mounted on little cylindrical bases which rise from 

 the side arm-plates. 



Description of an Individual. — Diameter of disk 43 mm. Length of arm 

 270 mm. Width of arm close to disk 5 mm. Mouth-angle covered with 

 thick skin and bearing at the apex an irregular clump of short, sharp spines, 

 like prickles. All the mouth-shields obscured by thick skin, except the 

 madreporic, whose transverse oval outline may be distinguished. It has a few 

 Irregular pores near its outer margin. Under aim-plates obscured by a thick 

 skin, on removing which they are found to be of a rounded quadrangular form, 

 wider without than within, swollen, and sometimes so cut out on the lateral 

 sides, where the tentacles pass through, as to assume an axe-shape. They are 

 continued upward by shapeless, rounded aide arm-plates, which appeal exter- 

 nally as well-marked spine-ridges, bearing a row of seven or eight small cylin- 

 drical knobs, each with a hole in its top to which is articulated a hooklet 

 The lowest knob is opposite the tentacle. No upper arm-plates; there are, 



♦ i(pis, snake ; ^pa\luv, arm. 



