54 
CEETACEOUS ECHIXODERMATA 
the total width of the joint cavity for the insertion of the arms is somewhat more than 
half the total breadth of the upper edge of one radial. At the sides, each of these 
plates has a broad smootliish or irregularly rugose slope extending towards the margin, 
and the united depression is continued as a narrowed triangle towards the centre of 
the respective upper suh-radial, and is margined on the angles by a more or less 
distinct smooth groove. All the plates are moderately convex, sub-mammillate in 
the centre, and ornamented with radiately arranged granular tubercles, or they are 
nearly smooth, merely with an indistinct radiating striation. Sometimes sharpened 
ridges proceed from the centre to the middle of the margin of each plate, hut they 
appear to he equally often wantmg. 
Some doubt exists as to these variations referring only to one species of 
Marsiipites, or to three, as indicated by E'orhes. 31. Imcigatus was inoposed by 
Forbes for fig. 8 of pi. xx of Eixon’s Geol. of Sussex. The form of the upper 
sub-radials in this figure is remarkably regularly hexagonal, and as- there is 
a slight difference in the ornamentation of the plates, it is possible that the 
species in question is a well founded one. The two other species admitted are 
31. ornatus of Miller and 3Iilleri of Mantell. When we compare the original 
figiu-es of these two forms, the only difference of any value appears to consist in 
the stronger ribbing of the plates in the forinei-, as compared with the latter. It 
seems to me very probable that this diflcrence is only an individual one, but 
unless the authentic original specimens had been examined and properly com- 
pared, it would be vain to prove the invalidity of the distinctions between the 
two species, particularly when we find two forms, such as those indicated, also 
represented in extra-Euroircan cretaceous deposits. 
31. 3Iilleri is a common fossil in the Upper Chalk of England, Xorth France, 
and it also occurs in the beds with Bel. quadrata in 'Western Germany, and in upper 
cretaceous beds in Poland. 
Locality . — Xear Arrialoor, in a whitish sandstone. 
Formation . — Arrialoor group. 
2. Marsupites covf. ornatus, 3Iiller. PI. 'V^IT, Figs 42 — 43. 
There are two plates of upper sub-radials evidently belonging to a species which 
differs from the former in a similar respect, as I have noticed to exist between ornatus 
and 3Iillcri. Both of the 2 )lates have the centre more distinctly mammillated, the 
ridges proceeding to the median parts of each side more distinct, and the interspaces 
between them more coarsely and less regularly granular. They evidently belong 
to ornatus, as iircsently understood, or else to a very closely allied siiecies. 
Locality, Near Olapaudy, in a yellowish ferruginous sandy rock. 
iormation , — Arrialoor grouj). 
( 124 ) 
