402 Mr. W. P. Sladen on the Structure of 



calcareous plates, which are very conspicuous on account of 

 the form and the arrangement which they present: those 

 situated within the central half of the abactinal surface are 

 disposed after the manner of the tessellation of an Ophiuroid 

 disk, whilst the outer portion of the pentagon is occupied by 

 plates which belong exclusively to the radial series, and re- 

 present highly modified brachial plates of very abnormal 

 development. The margin of the test is surrounded by^ a 

 close-set fringe of small compressed spinelets ; and the sin- 

 gular appearance of the organism is still further enhanced by 

 the possession of very short rudimentary rays of Ophiuroid 

 type, which extend from the angles of the disk in continuation 

 of the radial series of plates. These diminutive arms, how- 

 ever, are quite insignificant both in size and character, in 

 proportion to the general dimensions of the disk ; and, as will 

 be pointed out more fully hereafter, they would seem, when 

 considered in relation to function, to be little more than aborted 

 members. 



On the actinal side, the mouth is central and surrounded 

 by ten large mouth-plates, which bear papillae only ; jaws, 

 jaw-plates, and teeth being wanting. In one of the interradii 

 there is a large escutcheon-shaped plate, homologous in its 

 position with the mouth-shield of an Ophiuran ; it bears a 

 puncture, and is probably the madreporite ; no other shields 

 are present in the remaining four interradii. The ambulacra 

 are largely developed and conspicuous, the tentacle-pores being 

 separated by thin plates and guarded by long lanceolate ten- 

 tacle-scales. The median line of the radii is occupied by a 

 series of large quadrate plates, which extend from the oral 

 rimse and represent under arm-plates, or what may perhaps 

 be more correctly spoken of as subambulacral accessory 

 pieces. The extensive triangular interradial areas are covered 

 with a scaling of small hexagonal plates, which diminish in 

 size as they recede from the actinostome ; and the plating ter- 

 minates at a short distance from the edge of the disk, leaving 

 a portion of the radial plates exposed, in the form of a narrow 

 marginal border. 



Description of a Specimen. Abactinal Aspect. — The upper 

 surface of the disk is regularly tessellated with clearly defined 

 symmetrical plates arranged in the following manner. One 

 irregular heptagonal plate, apparently the representative of 

 what would normally be a pentagonal one, occupies the centre, 

 and is encircled by seven slightly elongated plates, irregularly 

 pentagonal and hexagonal, which are the representatives 

 of a normal series of five, the splitting of two of these having 

 given rise to the present irregularity, to which is also due the 



