102 BRITISH PALEOZOIC ASTEROZOA. 



the only other conclusion is that it is a large intermarginal. This conclusion 

 seems to be borne out by examination of the other interradial areas, which 

 appear to be filled up by somewhat smaller intermarginalia. There is another 

 piece of evidence which suggests that the form has gone beyond the " Hudsoruister" 

 stage. This is the appearance of the first pair of supero-marginalia which are 

 thrust somewhat apicalwards, as in some of the Mesopalgeasterinse. 



The infero-marginalia having been accurately determined, it is comparatively 

 easy to orientate the remaining plates. The supero-marginalia on the right side 

 of arm v are, of course, the ossicles which are above the infero-marginalia, and 

 the corresponding series can be traced on the other side of the arm. Radialia 

 and adradialia appear to be present, although the arrangement is not very 

 regular. The supero-marginalia on both sides of arm iv can also be identified. 

 Between these two series there is oiih/ one row of rather scattered ossicles. 



The appearance of the apical surface of arm i also suggests that only broken- 

 down radialia are present here, and that there are no adradialia. The supero- 

 marginalia on the right side of the arm are somewhat scattered, because the 

 row of the infero-marginalia, which are exhibited in full length, have been 

 thrust upwards. A corresponding row of supero-marginalia may be seen on the 

 left side of the arm. Between them there is but one row of small scattered plates. 

 Arm II, on the other hand, suggests that both radialia and adradialia are present, 

 and the very varied aspect of the arms, and the want of regularity in the plates, 

 show the form to have an unsettled structure, and to be in a critical stage in its 

 lineage history. 



A further interesting point is the passage of the infero-marginalia upwards 

 to form the apical boundary of q,rm v as it is followed distahvards. The supero- 

 marginalia tend to lose themselves in the mass of plates in the apical region. 

 This arrangement is strongly reminiscent of that in Urdiidstcr [viile p. 107). 



Measurements. — R : r : : 9 mm. at least : 3"5mm. 

 Width of arm i at base, 2-2 mm. 

 II 2-0 



v 0-2 



Oniaiihi'iif. — The ossicles appear to be bare of ornament. 



Horizon and hoc.alitij. — Middle Silurian (Wenlock Limestone) of Dudley. 



(2) Mesopalceaster (r) Iclleyi bears a general resemblance to Neopalseaster ciaio- 

 fordfivillevsis. Miller, especially in the form and arrangement of the primary plates 

 of the disc. It differs, however, in the fact that in the latter form " each supra- 

 marginal plate lies wholly and directly over an inframarginal, and the pieces 

 appear as one consolidated plate with the abactinal side convex and the actinal 

 nearly flat " (Schuchert, 85, p. 137). The two forms differ widely in horizon, 

 Neopalxaster cratvfordsvillensis being found in the crinoid beds of the Keokuk 



