PLATYCRINIDiE. 721 



mal face, having only a small angular- projection at the median line. Dis- 

 tichals two, short, four times as wide as high, those of the same ray 

 suturally connected as far as the middle of the second plate, which latter 

 gives off an arm to the outer side of the ray. The succeeding orders of 

 brachials form the dorsal side of the appendages ; they are composed of an 

 indefinite number of successive orders of two plates each, of which every 

 second jDlate is an axillary, thus giving off alternately from the one side an 

 arm, and from the other the next order of brachials. The arm-bearing faces 

 are much the shortest, and slope abruptly downward ; the others form a 

 nearly horizontal line, so that the plates of the appendages are transverse 

 but not exactly parallel, as the axillaries are slightly cuneate. The trunks 

 are three to four times thicker than the arms ; they decrease but little in 

 size upwards, and terminate in two short arms; the plates are of nearly the 

 same length, about three times as wide as long, rounded on the back, and 

 transversely angular at the outer faces. The length of the arms cannot be 

 accurately ascertained from the specimens, but it appears as if the proximal 

 ones did not rise to the top of the crown ; they are biserial, of moderate 

 size, pinnule-bearing, and they decrease slightly in width upwards. The 

 proximal arm plates are rather deeply set into the ray, and while they rest 

 chiefly upon the cuneate axillary, they abut also against the adjoining plates 

 above and below, which are truncated for their reception. In one of the 

 specimens, the arms near the calyx touch five plates, but higher up only 

 four, which is the general rule. The arms are very numerous; in a speci- 

 men of medium size we counted twenty-four arms to one branch, but the 

 extreme end is not preserved; and we have reason to believe that they 

 averaged in large specimens at least ten more, which would make about 

 thirty-five to the half ray, or three hundred and fifty to the individual. 

 Beneath each arm, within the appendages, there are two well defined respi- 

 ratory pores, one piercing the upper edge of the arm facet and lower end of 

 the node-bearing plate above, the other placed at the angle formed by the 

 same plate and two adjoining brachials. 



Ventral disk depressed, the orals moderately large, almost central, and 

 the middle part of the plates elevated into rounded, papillate nodes with 

 roughened or wrinkled surfaces; the node of the posterior oral largest. 

 Interambulacral plates flat and without surface markings ; they generally 

 consist of four oblong plates, of which the three of the first row are exposed 

 in a side view, only their upper ends, Avhich bend abruptly inward, being 



