METOPASTER PARKINSONI. 37 
additional supero-marginal plate. It will be seen that there are nine supero-marginal 
plates, exclusive of the terminals (or so-called oculars) on each side of the penta- 
gonal disk. At first sight the presence of this odd intermediate marginal plate in 
association with the strikingly modified ultimate plates found in this example 
would appear sufficient to indicate a well-marked variety, if not actually a distinct 
species. A careful examination of the collection in the British Museum shows, 
however, that this assumption is untenable, for in another example (which bears 
the Museum number “ 46,765’), which also possesses an odd intermediate marginal 
plate on two of its sides, the ultimate plates are in no way specially abnormal in 
their form or size. The presence of the additional supero-marginal plate would 
in this case, therefore, appear to be only an occasional variation, and, so far as I 
am able to detect, occurring independently of other structural modifications. I 
have observed no differences either in the proportions or the ornamentation of the 
intermediate marginal plates worthy of remark. This view is strengthened, if not 
absolutely confirmed, by the presence in another specimen (British Museum number 
* 46,796’), drawn on Pl. XII, fig. 1 a, of nine supero-marginal plates on two sides 
only of the disk, the remaining three sides having the normal number of eight 
marginal plates. In this example the ultimate plates are distinctly larger and more 
produced than in the truly normal forms of the species, but it will be noticed that 
their development is unequal, as is also the case in No. ‘* 46,765; ”’ in other words, 
in three of the rays one of the ultimate plates is smaller than the corresponding 
ultimate plate to which it is adjacent. The two sides which have an additional 
marginal plate are also the sides which have one of the smaller ultimate plates, 
and the inference naturally follows that the additional plate is to balance or com- 
pensate for the smaller size of the ultimate plate ; and I am inclined to think that 
the additional plate is not in this case a true odd interradial marginal plate at all, 
such as occurs in Gnathaster, Sladen, but that it is only a supplementary plate 
formed because its primitive or embryonic rudiment has not been included in the 
series merged together during development to form the structurally compound 
ultimate plate. That these ultimate plates are compound, or formed by the union 
of several embryonic plates, there is in my opinion little doubt when regard is had 
to the embryonic history of the ultimate plate and the associated infero-marginal 
plates. 
