METOPASTER PARKINSONI. 33 
twice the length, the actual measurements in the specimen under notice being, 
length 4°75 mm. and breadth 9°5 mm. respectively, i.e. as 1:2. The abactinal 
surface of these plates is distinctly convex, with a slight depression along their 
margins of juncture, formed by a well-defined bevel along the sides and adcentral 
end. The general surface of the whole series is well rounded, the curvature 
being regular and uninterrupted between the adcentral margin of the plate and 
the margin in the lateral wall adjacent to the infero-marginal plates. The height 
of the plates as seen in the margin is a little greater than their length, and there 
is no diminution in height as the plates approach the extremity of the ray—in fact, 
the ultimate paired plate is not unfrequently higher than the other plates in 
consequence of a tendency to become gibbous on its abactinal surface. The 
whole superficies of the plates is covered with small, widely spaced, equidistant, 
uniform punctations, and there is a depressed border along the margin of the 
plate, varying slightly in breadth in different examples, covered with much 
smaller and closely crowded punctations, upon which much smaller granules than 
those upon the median area of the plate were originally borne. ‘Traces of these 
granules may occasionally be found wi situ. 
The ultimate paired plate is larger than any of the other supero-marginal 
plates, and is of a different shape. It is subtriangular in form as seen from 
above, and one margin touches the corresponding plate of the adjacent side of the 
disk throughout, the junction coinciding with ‘the median radial line of the disk. 
The length of this margin of the plate is subequal to or only slightly greater than 
the breadth of the preceding marginal plates. In small specimens the subequal 
measures are the rule, whilst in larger examples the plate becomes more elongate 
and produced in the direction of the prolongation of the ray. When viewed in 
the margin of the test the form of the ultimate plate strikingly resembles that of 
the carapace of some Coleoptera. The length of the plate in this aspect, measured 
from its outer extremity to the margin adjacent to the penultimate plate, is in 
small and medium sized specimens about once and a half the length of the other 
marginal plates, but in large examples it may be as much as, or even exceed, 
twice their length. The surface of the ultimate plate bears a similar ornamenta- 
tion to that on the other supero-marginal plates. 
The odd terminal plate is very small, appearing externally when denuded of 
granules like a truncate cylinder, having a fanciful resemblance to a cannon 
projecting from a porthole. This plate seems to be very rarely preserved in situ 
in the fossil state. In a remarkably good specimen belonging to the British 
Museum Collection (marked “ H 2034’) (see Pl. XVI, figs. 2a, 26) each of the 
terminal plates preserved bears at its outer truncate extremity a single horizon- 
tally placed entrenched pedicellaria. Whether this regularly placed pedicellaria is 
always present on the odd terminal plate in this species I am unable to say. 
