ECHINODERMATA. 135 



Diameter of disk 1!) millim., length of arm about 83 millim. from 

 the edge of the disk, width of arm at disk 6 millim., height of same 

 5 millim. Fourteen mouth-papillee of fair size, the outermost the 

 largest; four stout teeth; mouth-plates with six sides, the aboraJ 



the longest and the adoral the shortest, longest hardly longer than 

 broadest axis; accessory mouth-plates large, longer than broad, 

 straight within, convex without; side mouth-shields distinct, in- 

 clined to be triangular, broader within than without ; granulated 

 space between mouth-papillae and mouth-shield very small. 



Lower arm-plates at first wider than long, but varying a good 

 deal in shape ; proceeding outwards the adoral edge becomes shorter 

 and shorter as the side arm-plates encroach more and more upon the 

 ventral middle line. The upper arm-plates at the base of the arms 

 are very wide, about four times as wide as they are long, further 

 out they become narrower, and towards the tip of the arm are very 

 much encroached upon by the side plates ; as a rule there are seven 

 spines on the plates. Tentacle-scales two. 



This fine form stands nearest to P. marmorata, from which it 

 may be distinguished by (i.) the much larger radial shields, (ii.) the 

 wider arm-bases, (iii.) smaller number of arm-spines, (iv.) larger 

 accessory mouth-shield, and (v.) proportionately shorter arms. 



Port Molle (14 fms.). 



OPHIOPINAX, g. n. 



It appears to be necessary to establish a new genus for the re- 

 ception of the form hitherto known as Pectinura stellata, owing to 

 the remarkable and regular arrangement of the large plates on its 

 disk ; this, which is only obscurely seen in smaller specimens, he- 

 comes very prominent in such larger examples as were obtained 

 during the voyage of the ' Alert.' 



The genus may be defined as follows: — Two elongated genital clefts; 

 mouth-papillae, teeth. Accessory oral shields ; arm-spines delicate, 

 attached to the outer edge of the side arm-plates. Eadial shields 

 large, separated by several radial or interradial plates from one 

 another. All the disk, except the radial shields, is covered by 

 granules, but there are no spines or spinous processes. Teeth (in 

 four rows) stout, blunt. Side arm-plates extend on to both actinal 

 and abactinal surfaces ; side mouth-shields naked. 



The extraordinary development of the plates between the radial 

 shields, both of the same and of the neighbouring sets, appears to 

 have escaped the notice of Ljungman and Grube ; nor can it, I 

 imagine, have been noticed by Mr. Lyman, as he retains 0. stellata 

 of Ljungman in the emended genus Pectinura. 



At this time it appears convenient to redescribe and figure the 

 species. 



