220 ME. W. PERCY SLADEN ON THE 



lines, and vary from a quadrate to a subrhombic form according 

 to position. The beigbt and length are nearly equal, the latter 

 being usually the greatest, although in some specimens the pro- 

 portions may be reversed. Normally each of the supero-marginal 

 plates bears a short conical spinelet on its upper edge ; but not 

 unfrequently these become aborted into little more than tubercles 

 and are sometimes absent altogether, especially in those plates 

 which are innermost in the arm-angle, and sometimes also on the 

 penultimate plate of the ray. The terminal plate is moderately 

 large and prominent, with the dorsal surface slightly tubercular, 

 the adoral margin deeply indented in the median line, and the 

 lateral angles fully rounded. The whole plate is directed at a 

 slight angle upward in relation to the ray, a circumstance which 

 emphasizes the strongly upturned appearance presented by the 

 rays, the general habit of which seems always to be more or less 

 bent upward or backward. Three moderately robust sjnnelets 

 are borne on the terminal plate — one placed at the summit of the 

 tubercular swelling in the median line of the ray and directed 

 vertically upwards ; and two somewhat smaller ones, which 

 stand one on either side at the extreme antei"ior lower angles of 

 the plate and directed outward. The infero- marginal plates 

 usually correspond both in number and in length with the com- 

 panion superior series ; but sometimes a little irregularity occurs 

 at the extremity of the ray, and an additional plate may be pre- 

 sent. The infero-marginal plates are not so high as the superior 

 series, and are longer than high, excepting perhaps one or two of 

 tlie innermost plates of the angle. The ventral edge o£ the 

 plates is curved outward, which gives a festooned appearance to 

 the lower margin of the series ; and after the first two plates in 

 the arm-angle the succeeding ones have their proximal end 

 higher than the distal end, so that the series tapers off gradually 

 towards the extremity of the ray. Cribriform organs, one in 

 each interbrachial angle, rather broad and well defined ; structure 

 lamelliform {cf. description above). 



Ambulacral furrows wide, open, straight, tapering from the 

 iTioiith to the extremity, occupying nearly the whole of the 

 under surface of the ray. Adambulacral plates rather short and 

 broad, with the adoral and aboral extremities somewhat incurved, 

 leaving a small cavity between adjacent plates, which is filled in 

 with membrane. Each plate bears two short, sharply-pointed, 

 conical ambulacral spinelets, the adoral one being longest. The 



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