66 ON THE ECHINODERMATA OF THE 



often large near the mouth-shields ; and the junction of the two kinds of scaling at the 

 margin is sharp. 



A row of spinules is often (but not always) seen along the generative slit, and is 

 continuous with that seen above the arm. 



The mouth-shields are distant from the edge, and are boldly curved and almost 

 circular, except orally, where they are produced into an angle ; and there are many 

 variations of this shape. The side mouth-shields are small, swollen and enlarged 

 where they impinge against the first side arm-plate ; and they end between the first two 

 tentacle-openings visible under the arm. They are united orally in some instances ; but 

 as a rule the union is not perfect ; and each carries a broad short scale which protects 

 the large first arm-tentacle, and which may be divided into two or even three parts. 

 The jaws are often slightly separated in front of the side arm-plates ; and they are 

 swollen just within the large and often triangular-looking jaw-plate. There are four 

 or five mouth-papillae on each jaw-margin, the aboral being broad and short, and the 

 others more or less spiniform or spear-headed. A large papilla is below the true 

 teeth on each jaw-angle ; and it and one on either side are to be seen on the jaw- 

 plate. 



The tentacular openings are very large ; and the first under the arm opens, as it 

 were, along the side of the jaw. The tentacle-scales are few, and like broad plates- 

 there being not often more than two on the side arm-plate, and two or three on the 

 side mouth-shield. 



The special characters of Opliiocten sericeum, as given by Lyman, are : " Arms three 

 to four times the length of the diameter of the disk ; two upper spines much the 

 longest ; papillae of arm evenly continuous, 1-4 upper arm-plates bearing papillae." 



The main variation which we have noted in the Arctic specimens of this species 

 consists in the greater length of the arm-spines as compared with those of more southern 

 examples. In a specimen 9 - 2 millims. in disk-diameter the length of the upper arm- 

 spine of the sixth joint was T85 millim. (in one case 2' 3 millims.) ; in another, with a 

 diameter of disk of 8'5 millims., the same spine was 1*8 millim. long, three arm-joints 

 in this individual being exactly 2 millims. In addition to the above, variations occur in 

 the contour of the mouth-shields ; and in the larger examples considerable irregularity 

 is also found in the number and position of the mouth-papillae. Amongst this collection 

 are several specimens having a very decidedly pentagonal form of disk. In our opinion 

 none of the above variations can be regarded as of greater morphological significance 

 than growth-phases, or, at most, than individual variations. The largest specimen 

 obtained was 1] millims. in disk-diameter. 



Numerous specimens were collected by both of the naturalists of the Arctic 

 Expedition ; and one set was collected by Capt. Feilden in Discovery Bay, 25 fms., hard 

 bottom, Cape Frazer, 80 fms., and Hayes Point, 35 fms., bottom temperature 29-5. 

 The others were in Mr. Hart's collection, from Discovery Bay, 15-20 fms., muddy 

 bottom, also at 11 fms. ; Franklin-Pierce Bay, 13-15 fms. 



