﻿OPIIIURA OLIVACEA. 



23 



twenty millemeters being about an average diameter for the disk of an 

 adult ; from 0. oUvacea it is distinguished by its barred arms and varied 

 coloration ; and from both of these by having no naked scales at the 

 base of the arm. 



LIST OF SPECIMENS. 



OpMura olivacea Lyman. 



Opliioderma oUvaceum AvuES ; Proceed. Boston Soc. Nat. Hist., IV. p. 134. 1852. 



SjJecial Marlis. — Uniform greenish above, without bands on arms ; 

 lower arm-plates nearly square, with outer side curved ; arms broad 

 near base, quickly tapering ; side mouth-shields naked. 



Description of a Sj^ecimen. — Diameter of disk, 11°""-; from outer 

 edge of mouth-shield to outer corner of opposite mouth-slit, 4.7°""' ; width 

 of arm without spines, 2.2""'- ; length of arm about 52™"- ; distance fi^om 

 outer edge of mouth-shield to inner points of teeth, to that between 

 outer corners of mouth-slits, 2.3 : 2.2. Mouth-papillse sixteen, outer one 

 small, next four stout, squarish, rounded ; innermost one longest, flat, 

 and tapering ; the rest like innermost one, but smaller. Teeth five or 

 six, similar among themselves, fiat, tapering, pointed. Mouth-shields 

 heart-shaped, with a truncated angle turned inward, and inner sides a 

 little re-enteringly curved ; length to breadth, 1.5 : 1.5. Side mouth- 

 shields small, and almost wholly naked, connecting innermost under 

 arm-plate with mouth-shield. Under arm-plates squarish, rather broader 

 than long, pretty strongly overlapping, bounded without by a long 

 curve, within and on the sides by nearly straight lines, which are, how- 

 ever, slightly re-enteringly curved, length to breadth (twelfth plate), 

 6:8. As the plates overlap a good deal, the inner laterals are very 

 short, while the outer laterals form a common curve with the outer side ; 

 hence comes the characteristic, even, square look of the plates. First 

 plate narrow oval, with a projection without, separating a pair of pores ; 

 next three or four plates with rather longer inner laterals, and more 

 curved outer sides, than those beyond ; about two thirds out on arm, 

 plates no longer overlap, so that inner laterals are well developed, givhig 



