146 BULLETIN OF THE 



Ophiacantha cosmica sp. nov. 



Plate X. Figs. 261 - 264, 262-266, 269, 270. 



Special Marks. — Eight or more feebly thorny arm-spines. Side arm-plates 

 only moderately prominent. Disk crochets rather coarse. 



Description of an Individual (Station 157). — Diameter of disk 18 mm. 

 Length of arm lOQ mm. Width of arm without spines 4 mm. Seven mouth- 

 papilla) to each angle, whereof the innermost is broad and flat, like the teeth ; 

 the others are short, pointed, and very stout ; outside these, and a little higher 

 in mouth-slit, is often a rounded scale of the second mouth-tentacle. Besides 

 these, there are, on the mouth-frames, from two to four small, peg-like papillaj. 

 Mouth-shields wide heart-shape, with angle inward ; length to breadth 2 : 3.5. 

 Side mouth-shields rather narrow, somewhat curved, meeting within, closely 

 joined to surrounding parts. Under arm -plates wide pentagonal, with outer 

 side cun'ed, and laterals a little re-enteringly curved. Upper arm-plates wide 

 fan-shaped, -with an angle inward, which in the basal plates is truncated. Side 

 arm-plates stout and moderately prominent ; meeting below, near base of arm, 

 but not above. Towards end of arm they meet above and below, and are more 

 prominent, so as to give it a somewhat knotted look. Disk full, and rising con- 

 siderably above the arms ; densely and evenly set with small stumps, which, being 

 freed of skin, are seen to be composed of five or six thorns, of different lengths, 

 soldered side by side ; these stumps appear also on the first two upper arm- 

 plates. Radial shields and disk-scales hidden. Eight even, translucent, cylin- 

 drical arm-spines, which taper to a blunt point and are under the microscope 

 slightly rough but not thorny ; lengths to that of an arm-joint 3.5, 3.5, 2.8, 

 2.5, 2.5, 2.2, 2, 2 : 1.5. Tentacle-scales rather small, longer than broad, 

 pointed. Color in alcohol, straw. 



The specimen just described is unusually large for this division of the genus, 

 which leads one to think that the bulk of those now known are immature, and 

 therefore to be treated with all the more caution. The young of Ophiacanthcu 

 differs from the adult as follows : 1. the arm-spines are fewer, longer, and more 

 slender and thorny ; 2. the side aim-plates are much larger and more project- 

 ing, giving a strongly knotted or beaded look to the arm ; 3. the mouth-papillse 

 are often less numerous and more slender ; 4. the armature of the disk is more 

 delicate, and the stumps or spines more thorny. Thus, a younger specimen 

 from the same station had a disk of 6 mm. in diameter and an arm 33 mm. 

 long. There were seven slender arm-spines, the upper ones much the longest, 

 and bearing fine thorns on the lower part of the shaft ; the arms were decidedly 

 knotted, though not so much so as in some other species (e. g. 0. serrata). The 

 disk-stumps had a short trunk which supported a crown of four or five thorns, 

 and resemljled Fig. 205, of Plate X. 



Another specimen (Station ] 22) was a little larger than the foregoing, and re- 

 Bembled it. There were eight scarcely thorny arm-s])ines, (he three uppermost 

 much the longest, 3.3 mm. (PI. X. Fig. 262). The disk was closely and evenly 



