ARCTIC SEA TO THE WEST OF GREENLAND. 55 



founded on two fine specimens, in August 1878 *. But it became evident, on the recep- 

 tion of Danielssen and Keren's communication, that they had forestalled the genus 

 Lutkenia, which is, of course, a useless synonym of Oplriopleura. 



The following is the generic diagnosis of Ophiopleura : 



"The skin of the disk is hard, naked, smooth, and covers a rich plating of scales. 

 The arms are in like manner overlaid with a smooth, thin skin, which extends up on the 

 side spines and lets the arm-plates appear. The back of the disk has ten elevated ribs. 

 Mouth-papillae occupy both sides of the mouth-slits, and are flat, with rounded, 

 smooth margins. The teeth form irregular rows, are compressed, and have a broad 

 basis. Foot-papillae. Two genital clefts in each interbrachial space." 



The diagnosis published in the ' Annals and Magazine of Natural History,' August 

 1878, page 188, under the title of Liitkenia, is as follows: 



" Disk notched, covered with very small scales. Radial shields small, widely 

 separate. Mouth-papillae numerous. Tooth-papillse. Teeth resembling tooth-papilla; 

 in double series, with accessory knobs. Generative slits small, midway between 

 mouth-shields and margin. Accessory scales to tentacular openings ; tentacle-scales 

 numerous ; on mid arm two. Spines small, distant, irregular. Lower arm-plates very 

 broad and short within the disk, and small and triangular without. Side arm-plates 

 meeting below throughout, but not above. Upper arm-plates broad and keeled near 

 the disk." 



OPHIOPLEURA ARCTICA, Duncan^. Plate IV, Figs. 1-2 c. 



The disk is large, subcircular in outline, tumid above and at the sides, flat below, 

 and is notched over the arms (1-j^ inch in diameter). 



The arms are twice and a half as long as the disk is broad, come well within it, are 

 very broad within the disk, and considerably so until the second third of their length. 

 They are flat beneath, convex and almost keeled above near the disk, and less so distally, 

 tall at the sides, and generally triangular in outline. The arm-spines are very small and 

 few in number. The colour is white with a little brown. 



The upper surface of the disk and the interbrachial spaces, to the aboral edge of 

 the mouth-shields, and except the naked radial shields, have a stout flaccid derm, covered 

 with excessively minute scales. The radial shields are small, pear-shaped, narrow, and 

 angular within, where they slightly overlap, and broad and curved without, with a free 

 edge there ; they are very distant, and bound the incision for the arm on either side 

 (-!% inch long). Many small scales, some elongate and others extremely small and oval, 

 are situated between the radial shields and the arm. There are no radial scales with 

 spines ; and the generative plates are hidden. 



The mouth-shields are small (-$ inch long), about as broad as long, somewhat 

 pentagonal, broadest without, angular within, the aboral edge being nearly straight. The 



* ' Annals & Magazine of Natural History,' ser. 5, vol. ii. p. 188. P. Martin Duncan on " Liltkenia." 

 t Ibid. pp. 188 & 266. 



