gg BULLETIN 76, UNITED STATES NATIONAL MUSEUM. 



tlio upper pd<,'c of each ambulacral plate produced into a tlun lamina between 

 consecutive pairs of ampulla;, the free edge bearing a comb-Uke series of slender 

 but irregular teeth or spinelets. R = 56 mm.; r=14 mm.; R = 4 r. Breadth of 

 rav at base, 16.5 ram. (t^^pe). 



Descripiim.— The jm.xilla? alTord tl\e most evident difference betvt-een orna- 

 tissimws and californicus. In califomicus there is a considerable area around 

 center of disk in which the paxilla? are smaller and more crowded than on remainder 

 of disk and on rays, and paxillie of midradial regions are more irregular, at least 

 in arrangement, than along margins of ray. In the present form the large paxillse 

 extend nearly to center of disk, there being only a small area of small paxillse. 



The paxilla; of sides of rays are not in such regular rows and are not easily 

 diflerentiated from the midradial ones. About three or three and one-half trans- 

 verse series of paxillse correspond to two superomarginal plates at base of ray 

 (usually five in califomicus), about five at middle of ray and six or seven near tip. 

 Opposite suture between second and third superomarginal plates about twelve 

 or thirteen paxillse can be counted across ray to same point on opposite side 

 (eigiiteen to twenty in califomicus). Large paxillse at base of rays with fifteen 

 to eighteen peripheral and ten to fifteen central spinelets, wliich are much longer 

 than in califomicus, terete, with rounded or clavate tips. Tabulum of paxilla 

 fairly broad so that both central and peripheral spinelets appear spaced, giving 

 the whole an open floriform appearance. Farther along ray, one to six central 

 spinelets to a paxilla, and upwards to fifteen or eighteen peripheral. At very 

 end of ray the paxilla; are much smaller. 



Although superficially there is little ciifference between the paxillse of the mid- 

 radial hno and those along sides of area, the plates or bases of the paxilla; are very 

 different. If the dorsal integument is removed, treated with caustic potash and 

 viewed from the inner or coelomic side the outline of the plates is readily seen. 

 Those along the midradial line and on center of disk are subcircular or very 

 obscurely lobed, are without papulse between them, and imbricate strongly, espe- 

 cially on disk. In large specimens the plates are usually entire, and roundish, 

 but tend to become lobed adjacent to the papular areas. On the ray these same 

 plates become more eUiptical and irregular in outhne as well as arrangement. 

 Two or three series have no pajiula; between them. On either side of the mid- 

 radial line of entire plates as well as over the interradial triangle of disk the plates 

 are smaller, strongly lobed, and touch or overlap by these lobes, allowing the 

 papula; to pass between. On the ray each plate is longitudinally elongated, sub- 

 elli])tical with two short but abrupt lobes at either end. The plates of consecu- 

 tive transverse rows alternate so that the processes join every plate with four 

 others, two adorally and two distally. Between any two plates there are, as a 

 rule, two papula; in a longitudinal row, a single plate having sLx papute surrounding 

 it. On the disk these plates are less regular and frequently have as many as six 

 lobes. Occasionally the slender lobes appear as independent ossicles. In the 

 largest specimen examined the regular i)lates on ray do not touch one another, 

 but do so regulariy in a medium-sized example. On the adcentral side of the 

 madrcporic canal where it pierces the dorsal integument is a large convex plate, 

 hollowed on the side toward the canal; on either side of this, forming a sort of 



