Ophiuran Genera Ophiopleura and Liitkenia. 267 



on the upper surface of the disk, some 15 millims. long and 

 12 millims. broad, with spaces between them and a clear cen- 

 trum. These very prominent objects on a disk with a circular 

 outline are exceedingly striking. They are associated with 

 very small wide-apart radial shields. The moutli-papilla3 are 

 numerous, and three are beneath the irregular set of rows of 

 true teeth ; and the tentacle-papillge are numerous. The 

 "E-ibber" are not in the specimens of Liitkenia arcfzca, nobis ; 

 but the other details are visible, with slight and specific differ- 

 ences. What, then, are these ten " Ribber " ? I find that in 

 Liithenia the bod^'- is rather more pentagonal than circular in 

 outline ; but there is a swollen condition of the upper part of 

 the disk in situations corresponding with the sacs leading 

 downwards into the remarkably limited generative openings ; 

 this is all; and the radial shields correspond with those of 

 OpMopleura. There can be little doubt that the "Ribber" 

 are of secondary importance ; and therefore I give the dis- 

 tinguished Scandinavian naturalists their due, and withdraw 

 Liitkenia. 



The question now arises, is the species Ophiopleura horealis 

 of those authors identical with Liitkenia arctica ? or are 

 the structural differences sufficient to separate them specifi- 

 cally ? The following are the distinctions ; and I have taken 

 the opportunity of again studying the species I named, so as 

 to be doubly sure. The species from Smith's Sound has 

 no " Ribber ;" its disk-scaling is smaller than in the species 

 horealis ; and the outline is pentagonal instead of circular. 

 The upper arm-plates are more convex and more medianly 

 pointed in the arctic species ; and the second and third lower 

 arm-plates of the boreal form differ entirely. The other lower 

 arm-plates have the breadth, but the few within the disk have 

 not the aboral point of the arctic form. The jaws differ in 

 shape: the accessory pieces are not seen in Ophiopleura horealis; 

 and its tentacle-scales are differently arranged and are more 

 numerous in mid arm than in the other form. The arctic 

 forms have round tentacle-spaces, and the others have them 

 elongate, at the root of the arms. The mass of tentacle-scales 

 and accessory pieces at their base, in relation to the tentacle 

 at the side of the first lower arm-plate, are very strongly 

 marked in the form from Smith's Sound, but not so in that 

 described from the sea to the east of Greenland. Botli are 

 very fine forms and large; and the slight increase of dimensions 

 in the boreal type is not sufficient to explain the structural 

 differences. I therefore consider the species arctica to hold 

 good and the classificatory position to be as follows : — 



