36 FOSSIL ASTEROIDEA. 
fossils so named by him were given in Dixon’s ‘Geology and Fossils of the 
Tertiary and Cretaceous Formations of Sussex,’ London, 1850. One, if not 
more, of the specimens delineated in that work is now preserved in the British 
Museum. 
Variations.—In some examples the breadth of the border formed by the 
supero-marginal plates on the abactinal surface is greater in relation to the disk- 
area than in others, and this variety was noted by Forbes (‘ Mem. Geol. Surv.,’ 
vol. 11, p. 472). I have not been able to establish the relation of this modification 
with any other permanent morphological character, nor to associate it with any 
special locality or stratum, and I am therefore led to consider, for the present at 
least, that the variation in question is one affecting individual examples of the 
species independently of other structural modifications which would warrant 
recognition by name. 
Two other variations are to be noted in this species which are superficially 
much more striking, and either of them would, if only isolated examples were 
known, lend a strong temptation to the separation of their possessor from the 
normal form of the species. One of the variations in question affects the large 
ultimate paired supero-marginal plates. On comparing the examples drawn on 
Pl. X, fig. 1 and fig. 2a, with fig. 1a, Pl. XII, and fig. la, Pl. XI, it will be seen 
that the ultimate plates are relatively much larger than the adjacent supero- 
marginal plates and are more produced at the extremity; whilst in the specimen 
delineated in fig. 2a, Pl. XI, this modification is carried to such an extent that 
at first sight it would appear scarcely possible to beheve that this fossil belongs 
to the same species as, for example, fig. 2a, Pl. X. I have, however, been unable 
to find any other constant variation from what has been considered the typical 
form of Metopaster Parkinsoni associated with this modification in the size and 
shape of the ultimate plates ; and as the most complete gradation between the two 
extremes may be traced in the splendid series of specimens now preserved in the 
British Museum, all obtained from the same locality and the same horizon, no 
reasonable doubt can be entertained that the variation in question is of a 
comparatively trivial character, affecting the individual independently, and that it 
is not stamped by correlation with other structural modification with sufficient 
importance to justify the forms being separated from the species, or even a name 
being given to the variety. All the examples referred to in the foregoing remarks 
and figured in the plates accompanying this Memoir are from the Upper Chalk, 
and were obtained from the same locality near Bromley. 
The fine specimen with large and greatly produced ultimate plates drawn on 
Pl. XI, fig. 2a, is also an example of the second variation in the structure of this 
species, to which I have alluded. This manifests itself in the presence of an 
