225 



changed in shape by the animal who throws it off when dis- 

 turbed. Most of the specimens found had lost their disks. 



It was found by Lieut. Kurtz and myself at Fort Johnson, 

 Charleston Harbor, in March. It lives at low-water mark, 

 buried in the soft mud at the depth of six or eight inches, and 

 extending one or two rays up to the surface. 



O. atra. Disk large, very soft, convex, orbicular, lobulate, 

 with a notch at the insertion of each ray. The pair of plates at 

 the insertion of the ray are ovate, short and broad, and close 

 together, separated only within by three or four minute plates. 

 From the outer side of each of these large plates, a row of 

 minute, transversely-elongated plates passes obliquely to the side 

 of the disk, thus giving to the disk the appearance of being mar- 

 gined. The remaining upper surface of the disk is covered with 

 minute crowded scales which are smallest about the centre. 

 The sides and lower surface of the disk appear smooth to the 

 naked eye, from the minuteness of the granules which cover 

 them. The mouth-rays do not taper, but are abrupt and wide 

 exteriorly, and have three or four teeth only at each side. The 

 interbrachial plates are rhomboidal, the large one being always 

 very prominent, swollen, and white. The rays are large at 

 their insertion, from which they taper gradually to their extremi- 

 ties which are very slender. Each joint has three lateral spines 

 on each side, which in length equal one half the width of the 

 ray. 



Length of each ray 4 inches; width at insertion, 0.13 inches. 

 Diameter of disk, 0.4 inches. In measuring, the width of the 

 spines is included in this and the preceding description. 



When alive this species was of a very dark gray color, nearly 

 black, except the white interbrachial plate. The arms were jet 

 black above, except their extremities. The remarks as to fre- 

 quent loss of the disk, made of the preceding species, apply also 

 to this. 



This species was first found some years since by Lieut. Kurtz 

 on Maurice's Island, Charleston Harbor, and also this year by 

 him and myself at Fort Johnson. It occurred at low-water, 

 buried in the mud, as did the last species. Another, the O. 

 elongata, (Say,) Miill. has the same economy, and being very 

 abundant, gave me frequent opportunities of observing its 



PROCEEDINGS B. S. N. H. 15 NOVEMBER, 1852. 



