ARCTIC SEA TO THE WEST OF GREENLAND. 59 



freezing-point, 1 0> 3 Centigrade. It is distinguished from Ophiopleura arctwa by the 

 presence of 10 very definite rib-like elevations of the upper surface of the disk, and by 

 its large disk-scaling, and general greater robustness and dimensions of the whole body. 

 The species Ophiopleura arctica has a more pentagonal outline ; the upper arm-plates 

 are more convex and medianly more pointed ; and in shape the second and third 

 lower arm-plates differ entirely from those of Ophiopleura borealis. The other lower 

 arm-plates of this last species 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 borealis. And its tentacle-scales are differently arranged and 

 are more numerous in mid arm than in Ophiopleura arctica. 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. 

 Both are very fine forms and large ; and the slight increase of dimensions in the boreal 

 type is not sufficient to explain the structural differences. 



In estimating the importance of the remarkable condition of the true teeth in 

 Ophiopleura arctica it is necessary to remember the great variations of the teeth of 

 such a form as Ophioglypha texturata, Lamarck, sp. Hardly any specimens of this 

 species can be found with entire teeth, so constantly are they divided and conical. 

 The same remark holds good for Ophioglypha Sarsii, Liitken, sp. 



Description of the Illustrations of this Species on Plate IV. 



Fig. 1. The upper part of the disk and arms : natural size. 

 2. The under surface of the disk : magnified. 

 2 a. The teeth, on the jaw-plate, oblique view : magnified. 

 2 b. The teeth from above : magnified. 

 2 c. The teeth approaching : magnified. 



Genus OPHIOGLYPHA, Lyman. 



OpliioglypTia, Lyman, Proc. Boat. Soc. N. H. vii. 1860. 

 Ophiura, Forbes. 



Disk covered with unequal, crowded, naked, more or less distorted scales, some of 

 which are swollen. Radial shields naked and swollen. Teeth : no tooth-papillae. 

 Mouth-papilla? long within, but small and short near the outer edge of the mouth- 

 tentacles. Arm-spines few, arranged along the outer edge of the side arm-plates. 

 Tentacle-scales numerous ; the innermost pair of tentacle-pores shaped like slits, 

 surrounded by numerous scales, and opening diagonally into the mouth-slits. Side 

 arm-plates meeting nearly or quite below, but not above. A notch edged with 

 papillse over the arm in the disk. The genital slits starting from the mouth-shields. 



l2 



