﻿PENTAGONASTER MEGALOPLAX. 27 



collections have, following Forbes, been erroneously referred to Pentagonaster 

 lunatus, which are in reality examples of Pentagonaster megaloplax. This is 

 unfortunate, for the latter form has thus become comparatively well known under 

 the name of Pentagonaster lunatus, a name which they must now cease to bear, as 

 the real Pentagonaster lunatus is quite a different form, and there is no doubt 

 whatever either as to the type (which is preserved in Norwich) or the priority. 

 The differences between the two species will be further noticed under the descrip- 

 tion of Pentagonaster megaloplax. 



2. Pentagonaster megaloplax, Sladen. PL IV, figs. 2 — 4. 



Goniastee (Asteogonium) lunatus, Forbes, 1850. In Dixon's Geology and 



Fossils of the Tertiary and Cre- 

 taceous Formations of Sussex, 

 London, 4to., p. 353, pi. xxiii, 

 fig. 9 (non Asterias lunatus, 

 Woodward, 1833). 



Body of medium size. General form depressed. Abactinal and actinal areas 

 flat. Marginal contour stellato-pentagonal, the major radius measuring a little 

 more than once and a half the minor radius. Rays short and not greatly pro- 

 duced, tapering gradually to the extremity. Interbrachial arcs regularly rounded, 

 curving gradually from the tip of one ray to that of the adjacent ray, which gives 

 a distinctly lunate character to the disk. Margin of uniform thickness. 



The infero-marginal plates are only five or rarely six in number, counting from 

 the median interradial line to the extremity. They form a very broad border to 

 the actinal area of the disk in relation to its size, and the breadth is maintained 

 until near the extremity. The largest infero-marginal plates adjacent to the median 

 interradial line measure 7 mm. in breadth, and about G'5 mm. or nearly 7 mm. in 

 length ; they are consequently almost square. The proportion of breadth dimin- 

 ishes in the succeeding plates as they recede from the median interradial line. 

 The infero-marginal plates have a more or less pulvinate appearance actinally, 

 consequent on being rounded or bevelled at the edges ; and they are slightly 

 tumid in the margin. Their whole actinal surface is covered with large, well- 

 spaced, deeply sunken pits, in the centre of which is a slight eminence — a struc- 

 ture which has almost the character of a granule surrounded by a scrobicule (see 

 PI. IV, fig. 2 b). On the surface which stands in the margin the punctations 

 are fewer and more widely spaced on the upper half of the surface — that is to say, 



