METOPASTER PARKINSONI. 83 



twice the length, the actual measurements in the specimen under notice being, 

 length 475 mm, and breadth 9*5 mm. respectively, ?. 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 lengtli, 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 difierent examples, covered with much 

 smaller and closely crowded punctations, upou which much smaller granules than 

 those upon the median area of the plate were originally borne. Traces of these 

 granules may occasionally be found iii situ. 



The ultimate paired plate is larger than any of the other sujsero-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 

 granides 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 " E 2034") (see PI. XVI, figs. 2 a, 2 b) 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. 



