a new Genus of Echinodermata. 403 



modification of the contour of the central plate. These repre- 

 sent the " primary plates " of Ophiuridae. External to the 

 last-named series of plates follows a ring of ten more or less 

 symmetrical pieces : the five which stand in the median line 

 of the radii are subtriangular or somewhat arrow-headed in 

 outline ; and the intermediate or interradial ones are irregularly- 

 pentagonal. Each of the five radial plates, as well as the 

 single central plate, bears a small tubercle of slight elevation. 

 The radial plates have their apex directed outwards and in- 

 serted between two large subtriangular plates, which represent 

 the radial shields of a typical Ophiuroid disk. These touch, 

 but only for a very short way on the middle of their inner 

 sides, being again separated by the insertion on their outer 

 margin of the triangular first upper arm-plate. The interra- 

 dial space between the shields of two neighbouring rays is 

 occupied by a single large quadrate plate, the outer margin 

 of which is nearly in line with the outer margin of the radial 

 shields. The whole series of plates above described form a 

 circular area which occupies the central half of the pentagonal 

 disk, the remaining outer portion being filled up by extraor- 

 dinary radial plates, the abnormal development which these 

 attain furnishing a highly remarkable feature in this Echino- 

 derm, as the following description will indicate. 



Succeeding to the triangular first upper arm-plate, and 

 situated in the median line of the ray, follow a series of 5-7 

 rectangular plates, twice as broad as long, and which diminish 

 in size as they approach the margin of the d ; sk, where they 

 become modified in form and pass off, by a gradation of 

 stages, onto the rudimentary ray as the aborted representa- 

 tives of upper arm-plates. On either side of each upper arm- 

 plate is a long, narrow, band-like lateral plate, representing 

 a side arm-plate, the breadth being equal throughout and cor- 

 responding with the length of the upper arm-plate, whence 

 they extend to the margin of the pentagonal disk, each plate 

 in the series being consequently shorter than its more internal 

 predecessor. With the exception of the pair belonging to the 

 first, each side plate is connected with its own proper upper 

 arm-plate, and proceeds from it at right angles to the median 

 line of the ray ; this direction, however, is almost immediately 

 changed, and the plate is bent sharply, though only slightly, 

 outwards ; whilst corresponding with this flexure in the 

 pseudo-side arm-plates there is a slight downward depression, 

 with a graceful curve, of the whole lateral series, which gives 

 to the median portion of the ray a decidedly raised or gibbous 

 character; and the series as a whole suggests some resemblance 

 in appearance to the body of a trilobite. Although the pseudo- 



^29* 



