ixTHonrcTiox. 3 



principal scales, or plates ; namely, primary pinfes (PI. IT. 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 they cover the whole back of the disk, and are only six in 

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

 sfnelds,^'^ their size, length to breadth, shape, &c. ; these are plates, arranged 

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

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

 the disk, forming radiating ridges, or "rilis": — radial scales (Fig. 8, ;«), 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 iuter- 

 brachial space, — their size and shape. 

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



J. Arm-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 sj)ine is counted as the jirst. 

 k. Tentacle-scales^^ (I'ig- l-^^ <?)» their number, shape, size, &c. 

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



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



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



7. IIo\v DISTINGUISHED, specifically, from other species of the same genus. 



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



the Museum of Comparative Zoology at Cambridge. 



^° Scutella radialia. " PaplllaB ambulacrales. 



