﻿INTRODUCTION. 3 



principal scales, or plates ; namely, primary plates (PI. II. Fig. 5), which 

 lie on the back of the disk, one in the centre, and others arranged along ten 

 lines radiating from it ; they may be distinguished by their greater size ; in 

 the young animal tlie}' cover the whole back of the disk, and are only six in 

 number, — their number, size, shape, and position are described : — radial 

 shields,^" their size, length to breadth, sliape, &c. ; these are plates, arranged 

 in five pairs, a pair over the base of each arm (Fig. 10, /) ; among Astro- 

 phytidas they are covered with thick skin, and extend nearly to the centre of 

 the disk, forming radiating ridges, or " ribs " : — radial scales (Fig. 8, m), large 

 scales sometimes found outside the radial shields: — genital plates, or genital 

 scales (Fig. 4, n), which run along the genital slits, on the side next the inter- 

 brachial space, — their size and shape. 



i. Genital Slits (Fig. 17, o). 



j. Ann-spines (Figs. 1 and 14,/)), their special characters and their lengths, com- 

 pared with each other and with the length of the lower arm-plate. The 

 uppermost spine is counted as the Jirst. 



L Tentacle-scales^^ (Fig. 15, 9), their number, shape, size, &c. 



I. Color. The colors of the animal, when alive and when in alcohol. 



5. Variatioxs op the Species, in size, color, number of parts, &c. 



6. Localities. In what seas found, and by whom, depth of water, habits, &c. 



7. How DISTINGUISHED, Specifically, from other species of the same genus. 



8. List of Specimens now in the collections of the Smithsonian Institution, and of 



the Museum of Comparative Zoology at Cambridge. 



'" Scutella radialia. " PapiUa; ambulacrales. 



