﻿18 FOSSIL ASTEROIDEA. 



for determination (see PL VII, figs. 2 a, 2 b). The marginal plates resemble in 

 character those of the specimen just mentioned, and they are like them covered 

 with a very fine punctation, unlike that of the typical Nymphaster Coombii. There 

 are also fewer plates in that portion of the interbrachial arc which may be said to 

 belong to the disk than in Nymphaster Coombii, but as the example is smaller, this 

 may probably be only a question of age ; or it may, like the punctation of this and 

 the preceding example, be attributed to variation, which I am disposed to consider 

 a not improbable reason for the differences, when regard is had to the horizon 

 from which the fossils were obtained, and consequently the changed conditions of 

 existence in which those Asterids probably lived. 



3. This specimen (figured on PI. VII, figs. 3 a, 3 b) is from the Lower Chalk 

 of Glynde, Sussex, and I consider that its reference to Nymphaster Coombii is less 

 doubtful than that of either of the two preceding examples. The fragment repre- 

 sents a portion of the abactinal surface. The supero-marginal plates are large, 

 and are covered with the characteristic coarse punctation of Nymphaster Coombii 

 (see PI. VII, fig. 3 b). The supero-marginal plates of the two sides of a ray 

 meet in the median radial line from the very base of the ray, distinctly charac- 

 teristic of the genus Nymphaster. Comparing this example with the typical form 

 of the species, there appear to be a much smaller number of supero-marginal 

 plates in the interbrachial arc belonging to the true disk, and on these grounds I 

 hesitate from accepting it positively as an undoubted example of this species until 

 further material is forthcoming to throw light upon the amount of plasticity 

 which may be accredited to this species. 



2. Nymphaster marginatus, Sladen. PI. VIII, figs. 4 a, 4 b. 



Body of medium size. General form depressed. Marginal contour stellato- 

 pentagonal. Rays well produced, rather broad at the base, and tapering gradu- 

 ally to a pointed extremity. Interbrachial arcs deep and rounded, the sweep of 

 the curve from the tip of one ray to the tip of the neighbouring ray being of a 

 paraboloid character. Margin rather thin. 



The supero-marginal plates form a broad and massive border to the abactinal 

 area of the disk. There are six plates on each side of the disk counting from the 

 base of one ray to the base of the neighbouring ray. All the succeeding plates 

 along the ray meet the corresponding plate of the opposite side of the ray in the 

 median radial line. The abactinal surface of the ray is thus occupied entirely by 

 the supero-marginal plates throughout its length. 



