INVERTEBRATES. 279 



above, and about equal in length to the diameter of the disc. 

 Plates of the upper side of the arms and disc, convex, or even 

 tumid; near the disc those of the rays hexagonal, heptagonal, 

 or irregular in form, alternating, and consisting of about five 

 or six longitudinal rows, with a few much smaller intermediate 

 pieces. Farther out the rays, they gradually pass into two 

 mesial ranges of oblong, alternating pieces, with their longer 

 diameter parallel to that of the rays ; while on each side of 

 these, minute irregular pieces fill the space between them and 

 the marginal pieces. Towards the extremities of the rays, 

 these little intermediate pieces diminish in size, and at last 

 become obsolete, leaving only the two middle and outer, or 

 adambulacral rows. Ambulacral furrows, in apparently undis- 

 torted specimens, deep, and nearly or quite twice as wide as 

 the row of pieces on either side ; adambulacral pieces rather 

 thick and strong, and liable to present considerable differences 

 in their obliquity and breadth of surface exposed, in conse- 

 quence of the compression or distortion of the specimen. Plates 

 of the under side of the disc, very much smaller than the adam- 

 bulacral, closely crowded together, and owing to their imbrica- 

 ting arrangement, presenting much the appearance of the scales 

 of a fish; immediately on each side of the rays, they imbricate 

 towards the latter, but near the middle of the space between 

 any two ambulacra, the imbrication is inwards towards the 

 mouth, so that in tracing the rows parallel to their longer 

 diameter, across between the rays, they are found to describe 

 a nearly semicircular curve, with a slight angularity near the 

 middle. 



Near the extremities of the rays, the dorsal pores are seen 

 to pass between the ends of the two mesial ranges of oblong 

 pieces, but farther in towards the disc, they are more irregu- 

 larly distributed. Our enlarged figure 7 b, pi. 19, represents 

 these pores and the dorsal plates, as seen in one of the rays, 

 with the convex outer portion of the plates ground away, in 



