REPORT ON THE ASTEROIDEA. 163 



The marginal plates occupy the entire lateral region, and represent the whole thick- 

 ness of the animal, forming perpendicular walls regularly rounded above and below. 

 Along the rays, the supero-marginal plates meet in the median radial line, and form a 

 complete casing to the ray, which is well rounded, small, and tapers but slightly. The 

 supero-marginal plates are eight in number (or, counting a very small aborted one, nine), 

 exclusive of the terminal. The plates which fall in the margin of the disk proper have 

 the length about equal to their height, but in those along the ray the height is greater 

 than the length. 



The infero-marginal plates correspond in number and in length to the superior series. 

 In the interbrachial arc, along the disk proper, the height is about equal to the length, 

 and the plates are uniform in size with the superior series. Towards the extremity of the 

 ray the height diminishes gradually, and the length is greater than the height — a reversal 

 of the relative proportions presented by the plates of the superior series. The marginal 

 plates are smooth and bear no spines ; but when examined microscopically have the 

 appearance of being subgranular, and built up of a rather open network. The plates of 

 both series are convex outwardly or tumid in a very slight degree, by which means the 

 sutural divisions of the segments are clearly marked out, and a somewhat annulated 

 appearance is given to the ray. The terminal plate is large and conspicuous, appearing 

 somewhat tubercular when viewed in profile, and oval in contour when seen from above. 

 This plate bears three short and rather robust spinelets — one at the terminal extremity 

 of the plate, situated in the median radial line, pointing in the direction of the prolonga- 

 tion of the ray, and diverging but little from the horizontal. Below this spine, and at each 

 side of it, on the angle formed by the actinal edge of the plate and the terminal extre- 

 mity, is a somewhat smaller spinelet, pointing in the direction of the prolongation of the 

 actinal margin of the plate. 



Seven cribriform organs are present in each interbrachial arc. They are narrow and 

 well defined, and their structure is papilliform. (See PL XXVIII. fig. 8.) 



The ambulacral furrows are narrow and straight, almost completely closed in by the 

 overarching adambulacral plates and spines, the tube-feet, which are arranged in simple 

 pairs, being entirely concealed from view. The adambulacral plates are about half as 

 broad as long, but diminish in size as they proceed outwards, and form along the ray 

 triangular prominences projecting into the furrow. Each plate bears three or four 

 spines, rather short, rapidly pointed, more or less compressed, invested with membrane, 

 arranged in line along the furrow margin of the plate, and sometimes oblique to 

 the course of the furrow. The row of spinelets can be raised at a right angle to the 

 surface of the plate, so as to allow the tube-feet to be protruded. Traces of an 

 aborted secondary or external spinelet, represented by a mere granule, may be detected 

 at the adoral extremity of the adambulacral plate, away from the furrow series. 



The mouth-plates are moderately large, the inner margins which fall in the median 



