112 OPHIACTIS KREBSIL 



side ; this lobe is not so distinct on the basal plates as on those a little 

 farther out on the arm; length to breadth (eighth plate), A : .7. Scales 

 of disk all pretty small and thin, very irregular ; no rosette of primary 

 plates in the middle, but sometimes a single primary plate may be dis- 

 tinguished in the interbrachial spaces ; scales 

 near the margin, about 60 to a square mm. ; 

 scales of interbrachial spaces, below, very fine. 

 Radial shields regular, elongated, pear-seed shape, 

 partially separated by one or two narrow scales ; 

 length to breadth, 1 : .4. There are a few scat^ 

 tered, short spines along the disk margin, and 

 also towards the centre. Arm-spines five, near 

 the disk six ; rounded, somewhat tapering ; sec- 

 ophiactis Krebsii. (Lower si le ) Qud ouc froui tlic top largcst ; first aud third of 



A' A"' inner lateral siiles of under arm- i j ji • ji i • n 



plates; A', outer side; A"', lateral about tlic sauie sizc J thc Tcst growuig smallcr 



to the lowest one ; lengths of second and sixth 

 spines to that of under arm-plate (fifth joint), .4, .2 : .6. Tentacle-scale 

 one, similar to the mouth-papillae. Color, in alcohol : variegated liright 

 sap-green and whitish ; radial shields green, tipped with white ; arms 

 banded green and white. 



Variations. — Not so many individuals have only two or three arms 

 as in 0. JliiUeri ; the usual number is six. A young one had the fol- 

 lowing characters : diameter of disk, 1.5™°* ; length of arms, 6"""" ; upper 

 arm-plates triangular, with a more or less sharp angle, turned inward, 

 not separated by side arm-plates ; under arm-plates longer than broad, 

 with an angle within, lateral and outer sides nearly straight, corners 

 rounded ; otherwise it resembles the adult, even to the number of the 

 arm-spines. A large specimen had a disk of 4.2'"'°-, and arms of 20"™-. 

 The disk is said to grow as large as 7°"°'. The side mouth-shields com- 

 monly meet within. 



This species is common at St. Thomas (A. H. Riise, Mr. Krebs) and at 

 Santa Cruz (Mr. Orsted) ; I have found it at Cape Florida ; Eev. J. E. 

 Mills, at Key West ; and Professor Agassiz has dredged it as far north 

 as Charleston, S. C, outside the bar of the harbor. It lives on corals 

 and sponges, and has been found from low-water-mark to three flithoms. 

 It may be distinguished from its six-armed neighbor, O. Miilleri, by 

 having four mouth-papilla) instead of two, and having a slight lobe on 

 the outer edge of the upper arm-plate ; and from 0. virescens, to which 

 it stands very near, by regularly tapering arms and somewhat narrower 

 upper arm-plates. 



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