46 OPHIOGLYPHA ROBUSTA. 



scales ; they barely touch without, being separated on the outside by 

 the first upper arm-plate, and within by a large scale ; length to breadth 

 about 1 : .7. The row of papillse along each side of the notch in the 

 disk and the genital slit has about nineteen, of which seven or eight are 

 stout, short, conical, and situated at base of arm ; and about twelve are 

 small and flat, and run along genital slit ; besides these, there is a row, 

 or irregular group, of from two to seven papillae standing on each side 

 of the upper arm-plate, just outside the papilla3 of the notch. Arm- 

 spines three, rounded, tapering, sharp, rather stout, upper one consider- 

 ably the largest ; lengths to that of under arm-plate (tenth joint), .8, .5, 

 .4 : .5 ; at tip of arm, the lowest sj)ine is flattened, and has one or two 

 small hooks. Tentacle-scales small, thick, flattened, with a rounded 

 end ; those of the mouth-tentacles more rounded ; mouth-tentacles with 

 a row of four or five scales on the side next mouth-shields, and three or 

 four on the other ; second and third pores w4th two scales on the inner 

 side, and one or two smaller ones on the outer ; pores beyond, only one 

 scale. Color, in alcohol, grayish white. 



Variations. — A young one had the following character : diameter of 

 disk, 3""" ; length of arm, lO"""- ; first under arm-plate triangular, with 

 rounded corners ; other plates broad heartrshape, with a point inward. 

 Upper arm-plates separated by side arm-plates ; long heart-shape, with 

 a point within, and a clean curve w^ithout. Arm-spines more slender 

 than in the adult. In centre of disk an irregular rosette of primary plates. 

 Only one or two papilla) along outer edges of radial shields, and none 

 at all outside of them, on each side of the upper arm-plates. The pro- 

 portions of the disk to the arms may be as follows : 3 : 10, 6.5 : 24, 

 6.5 : 25.5, 7.7 : 32, 8 : 30. Liitken also gives 10 : 30 and 7 : 21. It is 

 very easy to get the arms too short, because they taper very finely, 

 and, if the point is broken off, it is not easily detected ; thus Dr. Ayres 

 gives the length of the arm (in broken specimens) as but little greater 

 than the diameter of the disk. Sometimes, in well-grown specimens, 

 the basal under arm-plates often touch each other, and the upper arm- 

 spine is proportionately more flattened ; these two variations are said 

 not to occur in European specimens. Large individuals have the under 

 and upper arm-plates more angular. The mouth-shields differ in the 

 proportional length, being sometimes regularly curved, or again having 

 two short lateral sides, and an outer curve. The color, in alcohol, is 

 often dark-greenish above, and ash-gray below. According to Dr. Liit- 

 ken, the living animal is dark gray, reddish, or violet, above ; often with 

 greenish arms, barred with dark gray ; the radial shields are almost 

 always light, and the mouth-shields commonly have a violet spot. 



0. rohusta differs from the other species of the genus in its short, 

 broad mouth-shields, and very finely tapering arms. I have examined 



