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OPHIOSTIGMA ISACANTHUM. 



outer side curved, and inner sides re-enteringly curved ; length to 

 breadth, .5 : .7. Side mouth-shields wider without than within, touching 

 y,„ 3 each other within for their whole breadth. Un- 



der arm-plates squarish, broader than long ; 

 bounded without and on the two lateral sides 

 by nearly straight lines ; within by an inner 

 side and two inner laterals ; length to breadth 

 (ninth plate), .3 : .5 ; first mider arm-plate dia- 

 I.) mond-shaped, and very small. Side arm-plates 

 '"' ""'"" """"' encroaching between the upper and under plates, 



but more between the former. Upper arm-plates oval diamond-shaped, 

 cleanly curved without, somewhat pointed at the lateral corners, and 

 with a rounded angle within ; leng-th to breadth 

 (fifth plate), .3 : .7. Spines of disk very short 

 and blunt, like elongated grains ; standing, not 

 close, but evenly. The outer ends of the radial 

 shields are indicated by slight swellings. Arm- 

 spines short, blunt, little tapering, evenly rounded, 

 of nearly equal lengths ; upper one a trifle the 

 stoutest ; length to that of under arm-plate, .4 : .3. 

 Tentacle-scales two, rather stout, longer than 

 broad, inside one larger. Color, when dry, white. 



Variations. — Dr. Liitken mentions a six-armed specimen. He speaks 

 of the color as commonly variegated brown, and mentions one black 

 individual. Say, on the contrary, described his specimen as white, with 

 green bands on the arms. An alcoholic specimen in the Cambridge 

 Museum is light brown. The arms seem commonly to be from four to 

 six times as long as the diameter of the disk. 



This rare species is said to be distinguished from 0. temie by the 

 form of the under arm-plates. It has been taken at St. Thomas, in four 

 fathoms of water (A. H. Riise) ; at St. John (Professor Prosch) ; and m 

 Florida (Professor Agassiz). 



LIST OF SPECIMENS. 



Whence obtaiued. 



Museum of Comparative Zooloijy. 



Florida. 

 Cape Florida 

 Florida. 



Prof. Ajiassiz. 

 G. Wurdeman. 

 Prof. Agassiz. 



