﻿OPHIOCOMA RIISEI. 



Ophiocoma Riisei Ltjtken. 



Ophiocoma Riinei LuTKEX. Vidensk. Meddelelser. Jan., 1856. 

 OjMocoma Riisei Ldtken. Addit. ad Hist. Opb., p. 143. 



Special Marks. — Arm-spines slender and even ; upper one consid- 

 erably the longest. Mouth-shields ovoid in outline, with the narrow 

 end inward. 



Description of a Specimen. — Diameter of disk, l.S.d""- ; outer edge 

 of mouth-shield to outer corner of opposite mouth-sht, 6"°- ; greatest 

 breadth of arm without spines, S""' ; length of arm, 63"""" ; distance from 

 outer edge of mouth-shield to inner point of tooth-papillae, to that 

 between outer corners of mouth-shts, 2.6 : 2.8 ; on each side four mouth- 

 papillae, of which that next the outer one is about twice as broad as the 

 others, which are more pointed and rounded ; four irregular horizontal 

 rows of rounded, crowded tooth-papillEe, three in each row, the lowest 

 row on a level with the mouth-pajjillte, and consisting of three (some- 

 times four) papillae, of which the middle one is the smallest, and aU 

 smaller and more flattened than those of the upper rows ; four flat, 

 square teeth, projecting beyond the tooth-pajaillae. Mouth-shields with an 

 outline nearly like that of an egg, the small end j^ointing inward ; length 

 to greatest breadth, 1.8 : 1.4 ; the outer end is not evenly rounded, but 

 makes a little projection to form part of the bridge between the inner 

 ends of the genital slits ; at their widest part the shields seldom fill the 

 Avidth of the interbrachial spaces. Side mouth-shields triangular, sol- 

 dered for their whole length to the sides of the mouth-shields, their 

 inner end about on a line with that of the mouth-shield ; length to 

 breadth, 1.2 : .8. Basal under arm-plates, and those in middle part of 

 arm, somewhat square, but having six sides, viz. : an outer side, long 

 and curved ; an inner, a little re-enteringly curved ; two laterals, long, 

 and re-enteringly curved to admit tentacle-scales ; and two inner late- 

 rals, of about the same length as the inner side. Farther out on arm 

 these sides are less sharply distinguished, the mner laterals becoming 

 merged in the curve of the inner side ; and at the tip of arm the plates 

 are elongated, and there are but four sides, an outer and inner, which 

 are curved, and two laterals, which are re-enteringly curved. Length 

 to breadth of plates : second, .6 : .8 ; third, 1 : 1.2 ; seventh, 1.2 : 1.6 ; 

 about two thirds the length of arm, .8 : .8 ; close to the tip of arm, .6 : .4. 

 Side arm-plates encroach at base of arm, on upper and lower plates, but 

 do not meet above tiU near the end of arm, and do not meet below 

 except at the very tip. Upper arm-plates pointed oval, but varying in 

 shape ; many, though somewhat oval, are angular ; first plate rudimen- 



