METOPASTER UNCATUS. 49 



outer margin and rises abruptly from a level area which occupies the inner half of 

 the plate along the margin touching the corresponding plate of the adjacent ray, 

 — the line of junction of the two plates coinciding with the median radial line. 

 The length of the ultimate plate is nearly twice that of the other marginal plates, 

 measuring 9'25 mm. in an example whose major radius is 36*5 mm. and minor 

 radius 27 mm. As seen in the marginal view of the test, the ultimate is not 

 higher or more tumid than the other marginal plates (see PI. XIV, fig. 2 b). The 

 outer margin of the ultimate plate has a slight concave curvature, and the inner 

 margin adjacent to the corresponding plate is curved convexly towards the 

 proximal end of the plate. In consequence of this rounding the two ultimate 

 plates in a pair do not unite throughout their entire length, but are separated by 

 a small notch at the end of the suture adjacent to the abactinal paxillar area of 

 the disk. On the small level area of the ultimate plate are a numljer of small 

 irregular tubercular eminences ; four or five larger than the others form a sort of 

 series parallel to the rounded mai'giu, and a longitudinal series of eight or nine 

 much smaller miliary granules run along the flank of the longitudinal tumidity of 

 the plate ; and several additional granules of intermediate size may be present in 

 the space between the two series just described. Excepting these granules, the 

 surface of the ultimate plate is smooth like that of the other supero-marginal plates. 



The abactinal area of the disk witliin the boundary of the marginal plates is 

 covered with small subregular plates or paxillar tabulae, an hexagonal form predomi- 

 nating. The plates in the median interradial areas are much larger than the other 

 plates on the disk, and a marked diminution in size in all the plates takes place as 

 they approach the margin. All the plates have their surface marked with a very 

 fine granulation. Small entrenched pedicellaritie are occasionally present, but 

 there appear to have been very few. 



The primary basal plates are larger than any of the other abactinal plates. 

 They are well seen in an example from the Upper Chalk of Kent, in which the 

 inner side of the abactinal wall is exposed by the removal of the actinal floor and 

 ambulacral plates. This specimen, which is preserved in the British Museum, and 

 bears the registration number " 35,496," is drawn on PL XI, fig. 3 a. The 

 example in question is further interesting in showing that the plates of the radial 

 regions have stellate bases, whereas the larger plates of the interradial regions are 

 sharply hexagonal, and fit closely to their adjacent plates (see fig. 3 h). 



The madreporiform body is very small, and is subsagittiform or irregularly 

 lozenge-shaped in outline ; in the example under notice it is embedded, all except 

 two straight sides, in one large adcentrally placed basal plate (see PI. XIV, 

 fig. 2 (1) ; the two straight sides are bounded each by one largo plate. The 

 surface of the madreporite is sculptured by very fine striations, which though 

 more or less wavy are directed subparallel to the adceutral sides of the body. 



