142 CLARK! EXTENDED RANGE OF GENUS LYDIASTER 



plates become shorter radially, so that they appear to be transversely 

 elongated; on the arms they slowly increase in size, though not in 

 regularity, the largest being something over 3 mm. in diameter; in the 

 distal half of the arm only one row of plates separates the superomar- 

 ginals, and the last four or five superomarginals are in contact along 

 the mid-dorsal line of the ray. 



Each of the abactinal plates is bordered by a ring of flattened gran- 

 ules, there being from 20 to 24 about the largest; in the central portion 

 of the abactinal surface each plate bears from two to six rounded 

 granules, each inserted in a small pit; toward the periphery these be- 

 come less numerous and less prominent, though they occur nearly to 

 the marginals ; the plates on the abactinal surface of the arms are some- 

 what more flattened than those of the disk, and the isolated granules 

 are less common in their central portion; most of the plates on the 

 disk bear spatulate pedicellariae with usually strongly dentate jaws, 

 each sunk in a deep depression, but on the rays, except for one or two 

 very small and imperfectly formed, these are lacking. 



The papular areas, which are confined entirely to the disk, are 

 very large; the only regions free from papulae are the center of the disk 

 and low triangles about three times as broad as high based upon the 

 superomarginals, in which areas also the plates decrease in size toward 

 the border and do not bear pedicellariae. The papulae are smaller and 

 less abundant in the mid-line of each ray than elsewhere, and this 

 causes the mid-radial plates in the peripheral half of the disk to appear 

 somewhat prominent. 



The marginal plates of both series are 24 in number; but the infero- 

 marginals are somewhat longer than the superomarginals, so that, 

 although the two series correspond in the interbrachial arcs, in the 

 distal part of the arms they alternate in position. The interbrachial 

 arc as defined by the superomarginals is straight, as defined by the 

 inferomarginals gently concave. 



In abactinal view the superomarginals, which are markedly tumid, 

 are seen to increase very slightly in width to the fifth, at the base of 

 the rays, thence decreasing gradually distally; in lateral view they de- 

 crease regularly in height from the center of the interbrachial arc, 

 where they are twice as high as the inferomarginals, to the distal third 

 of the arm, where the plates of the two series are of about the same 

 height. In the interbrachial arc the outer third of the superomarginals 

 is vertical, and the inner two-thirds bends inward so that the inner half, 

 which is flat, extends at an angle of about 45 degrees over the dorsal 

 (abactinal) margin ; on the arms the inner half becomes more nearly 

 horizontal ; here also the inner border of the superomarginals is straight, 

 but on the arms it becomes slightly convex or angular. On the aver- 

 age the superomarginals, as viewed abactinally, are about twice as 

 deep as long, those at the arm bases being somewhat longer, those 

 toward the arm tips slightly shorter. 



Each superomarginal carries on its surface numerous widely spaced 

 deciduous granules arranged in the interbrachial arc roughly in five 



