METOPASTER MANTELLI. 39 
there would appear to be only seven plates on the entire side, but only one 
radial angle is preserved intact, and much displacement of plates has occurred. 
The circumstance is im any case of comparatively trifling importance. The 
supero-marginal plates form a conspicuous and moderately broad border to the 
abactinal area of the disk, of uniform breadth throughout, and measuring about 
6 mm. at the median interradial line in an example whose minor radius measures 
about 25 mm. 
Excepting the ultimate paired plate all the supero-marginal plates are nearly 
subequal in size ; the plates, however, adjacent to the median interradial line are 
slightly longer in relation to their breadth than those adjacent to the ultimate 
paired plate. The plates adjacent to the median interradial line have the appear- 
ance of being nearly square in outline as seen from above, the actual dimensions 
in the specimen under notice, whose side measures 38 mm., being length 5 mm., 
and breadth 6 mm. Ina smaller example, with a side measurement of 29°5 mm., 
the corresponding plate is 3°5 mm. long and 4°5 mm. broad. In Forbes’s type 
the measurements are, length 3°75 mm., and breadth 5 mm. 
The abactinal surface of the supero-marginal plates is slightly tumid, and the 
general surface of the whole series forms a well-rounded regular curve from the 
adcentral margin to the margin in the lateral wall adjacent to the infero-marginal 
plates. The height of the plates as seen in the margin is less than their length, 
the actual measurement being about 3 mm. Their abactinal contour is distinctly 
convex, but not gibbous. The whole superficies of the plates is covered with 
small, widely spaced, equidistant, uniform punctations. In some examples which 
have been subjected to much weathering the punctations are almost obliterated, 
as in the case of the fine specimen shown on Pl. XIII, fig. 2a. <A narrow 
depressed border surrounds the margin of the plate, bearing very small, closely 
crowded punctations, those adjacent to the main or median area being in serial 
arrangement. Occasionally a small entrenched pedicellaria may be detected on 
the median area of the plate (see Pl. XIII, fig. 2 0). 
The ultimate paired plate is small and triangular in outline 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 the plate—that is to say, of the side of the plate which falls in the margin 
of the disk—is a trifle greater than the length of the largest supero-marginal 
plate, measuring in the example under notice nearly 5°5 mm., whereas the 
breadth of the plate, or measurement of the side adjacent to the penultimate 
supero-marginal plate, is not more than 5 mm. Near the outer or apical extremity 
of this plate when seen from above there is frequently a more or less strongly 
developed tendency to gibbosity present. 
The abactinal area of the disk within the boundary of the marginal plates is 
