METOPASTER PARKINSONI. 87 



additioual 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 PI. 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 interrndial marginal plate at all, 

 such as occurs in Gnathaster, Sladeu, 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. 



