256 



patch in the centre ; rest of upper surface having a ground tint 

 of Hght, yellowish brown, slightly clouded with olive ; arms, 

 above, dirty white, irregularly barred with yellowish brown, 

 each bar usually including two or three joints ; lower surface of 

 disc and of arms uniform dirty white, with a brown tint just on 

 edges of disc. 



Variations. According to drawings in possession of Prof. 

 Agassiz, the color is almost as variable as in 0. appressa. A 

 common pattern is a dark green disc with a blotch of white in the 

 centre, and arms banded with darker and lighter green. Speci- 

 mens with a disc of 14 millim. have the lower arm-plates propor- 

 tionately longer and more octagonal. 



This species resembles both 0. hrevispina and 0. olivacea, 

 but is larger than either, 20 millim. being about an average for 

 the disc of an adult ; it is further distinguished from the former 

 by stouter arms and the different proportions of the lower arm- 

 plates, and from the latter by barred arms and varied coloration. 



Charleston, South Carolina. Prof. Agassiz and Prof. F. S. 

 Holmes. 



Museum of Comparative Zoology, Cambridge. 



Ophiocoma, (Agas.) 



Opliiocoma Alexandria (Lyman.) 



Description of a Specimen. Diameter of disc, 17 millim. 

 Length of arm, 120 millim. Mouth-papillse flat, rounded, of 

 nearly equal size, rather small; 9 or 10 to each angle of mouth. 

 Tooth-papilla3 about 9, crowded, bead-like, most of them in two 

 vertical rows. Teeth 3 or 4, stout, flat, a little tapering. Mouth- 

 shields nearly round, with inner end slightly truncated ; length to 

 breadth, 1.6 : 1.6. Side mouth-shields very narrow and small, 

 occupying the inner portion of the mouth-shield, but not meeting 

 within. Under arm-plates very regular ; not overlapping ; 

 rounded octagonal ; inner angles less rounded than outer ones ; 

 the plates within the disc a good deal smaller than those beyond ; 

 length to breadth, (20th plate,) 1.2 : 1. Upper arm-plates regu- 

 lar, oval heart-shaped, with the point inward ; length to breadth, 

 (9th plate,) 1.2:2.2. Grains of disc, under the microscope, like 

 very short, blunt spines, about 25 to a square millim., of the 



