64 FOSSIL ASTEROIDEA. 



radial line the length at the outer and at the adcentral margin is equal, but in the 

 second plate the length at the adcentral margin is greater than at the outer 

 margin in consequence of a slight extension of the plate to make up for the 

 diminution in size of the ultimate plate, the distal lateral margin of the second 

 plate appearing to be slightly hollowed out for the reception of the ultimate plate, 

 and the corresponding second plates of two adjacent rays consequently touch at 

 their adcentral ends. The ultimate plate is triangular or wedge-shaped in form, 

 and its breadth is less than that of the other supero-marginal plates, and the 

 corresponding plates of the two adjacent sides touch one another thoughout, the 

 line of junction falling in the median line of the ray. The ultimate plate is not 

 unfrequently a little longer at the outer margin than the other supero-marginal 

 plates. 



The abactinal surface of the plates is regularly convex along the line of 

 breadth, almost resembling the segment of a cylinder, and no special gibbosity is 

 developed. The abactinal area and the lateral area of the plate form a right angle, 

 and the uniting curve is short and abrupt. The height of the supero-marginal 

 plates, as seen in the margin, is less than their length, the height being 2'25 mm. 

 where the length is 2"75 mm. There is no diminution in height as the plates 

 approach the extremity of the ray, and no special prominence is noticeable in the 

 ultimate plate. The abactinal surface of the plates is ornamented with irregular 

 and frequently elongate eminences or ridges, which are usually disposed trans- 

 versely or in the direction of the length of the plate. Two or three series may 

 be present, as in PI. XVI, fig. 4, or the linear ridges may be bent at a right 

 angle, or otherwise curved — a character which is probably the result of several 

 eminences being merged together, by which means a peculiar hieroglyphic-like 

 marking is produced, which fancifully resembles Chinese writing to a certain 

 extent (see PI. XVI, fig. 5 c). This character is emphasised in weathered 

 specimens. The ornamentation covers the whole abactinal area of the plate, but 

 does not extend upon the lateral wall. Sometimes the form of the eminences is 

 more rounded and tuberculiform, as in Mitraster Hunteri; this is distinctly the 

 case in one example, and minute punctations are present in the channels which 

 intervene, but it is to be noted that the tubercles and pits cover the ^vhole 

 abactinal area of the plate, and that there is no gibbosity, with its outer flank 

 devoid of tubercles, as in Mitraster Hunteri. There is a very narrow depressed 

 border round the entire margin of the plate, from which the regular convexity of 

 the plate rapidly rises. The border is very minutely pimctate, and there appears 

 to be usually only a single regular lineal series of punctations. The ornamentation 

 of the ultimate paired plates is similar to that on the other supero-marginal 

 plates, but is more rounded and tubercle-like even in an example in which the 

 broad lineal ornamentation occurs on the other plates. 



