H*w.<t^f,v I 



257 



same character below, but less numerous. Arm-spines 5, robust, 

 rounded, somewhat flattened, a little tapering, blunt ; third spine 

 from the top longest ; the longest spines are near the 20th joint ; 

 lengths to that of under arm-plates, (18th joint,) 2, 2.2, 2.7, 1.8, 

 1.5 : 1,2 ; two lowest spines more slender than the upper ones. 

 Sometimes a short, supplementary spine above. Longest spine 

 on this specimen, 3.2 millim. . Close to the disc there are often 

 6 spines, which, however, are smaller than those beyond. Ten- 

 tacle-scales large, rounded oval ; on first two or three joints 2, on 

 the rest only 1. Color, in alcohol: above, disc uniform gray 

 brown ; arms yellowish brown, barred with darker ; some of the 

 upper arm-plates finely marbled with lighter ; below^ several 

 shades of light brownish yellow ; a light brown stripe running 

 along the lateral sides of the under arm-plates. 



Variations. The pattern of the color varies little, but the 

 ground tint may have more of yellowish gray or of brown. The 

 proportions of arms to disc vary from about 5 to 9 times, ac- 

 cording to age and other circumstances. Very large specimens 

 have the disc 22 millim. in diameter, and 6 or 7 spines on most 

 of the arm-joints. A young one had 4 arm-spines and only 2 

 tooth-papillae ; the disc was covered with imbricated scales and 

 only a few large grains ; the disc was to the arms as Z.2 : 22. 



0. Alexandri was first received from my friend, Mr. Alex. 

 E. R. Agassiz, after whom I have named it. It resembles 0. 

 pumila, but differs in having the 3d (or 4th) arm-spine much 

 longer than the rest. 



Acapulco, West Coast of Central America. Alex. E. R. Agas- 

 siz. 



Museum of Comparative Zoology, Cambridge. 



Cape St. Lucas, Lower Cal. J. Xantus. 



Smithsonian Institution. 



ADDITIONS AND CORRECTIONS. 



OpMura teres has radial shields until it attains a considerable 

 size. The specimens before described (page 198) were very 

 large, and had the disc completely covered with grains. Four 

 smaller specimens, since received, three from Mr. A. E. R. Agassiz, 

 at Panama and Acapulco, and one from Mr. Xantus, at Cape St. 

 Lucas, all had radial shields. The largest of these had a disc of 



PROCEEDINGS B. S. N. H. VOL. VII. 17 JUNE, 1860. 



