MIMASTER, A NEW GENUS OF ASTEROIDEA. 583 
by the dorsal covering of the new species, the abactinal portion of the skeleton 
consists of a rather closely reticulated calcareous framework, built up of small 
imbricating plates, upon certain of which the paxillz are borne. The skeleton 
of Mimaster, on the other hand, is constructed quite differently ; the whole 
abactinal floor being composed of paxillee alone, each of the paxillee consisting 
of a pedicel, with a slightly expanded base and a rounded clavate extremity, on 
which the spinelets that form the crown are articulated. The expansion of the 
base of the paxillz is very slight, subcircular, or irregular in outline, and usually 
exhibiting two or three faint prolongations. The bases of the paxille are 
closely placed, and occasionally a trace of overlapping may be found here and 
there. This structure is identical with that met within Astropectinide, and has 
hitherto been looked upon as specially characteristic of that group. 
The adambulacral plates, the ambulacral spines, and the mouth-plates have 
strictly the characters of the Gonzasteride. The marginal plates are likewise 
suggestive of the same group, and, notwithstanding their inequality and in- 
significant development, approach the habit of such forms as Astrogonium 
pazillosum (Gray), from which also the general outline of the body is not far 
removed. 
The ventral plates recall in their character the ventral plates of Asterinide, 
- whilst their arrangement also approaches in a certain degree that of some of 
the Gontasteride. 
Possessing such a great complexity of structural character, Mimaster natu- 
rally stands in a very isolated position. For the present I propose to rank the 
genus provisionally amongst the Gonzasteride, but reserve any expression of 
opinion as to its definite position in that group until an opportunity is afforded by 
a further supply of material for studying more closely the anatomy and general 
structure. We may even say that the structural formula of the genus does not 
appear to admit of close association with any form at present known. Radi- 
aster, a genus established by M. PErrier* (from a specimen obtained by the 
U. S. Coast Survey steamer “Blake,” in the dredgings in the Gulf of Mexico 
and the Carribean Sea), appears to possess characters in common with our form, 
and may probably prove to be one of its nearest allies. As far as their indi- 
vidual relations are concerned, the description given by M. PErrirr indicates 
important differences, which separate the forms widely. Without drawing any 
closer comparison than that afforded by the short description of my learned 
colleague, the following points may be mentioned. Radiaster possesses only 
one series of marginal plates; each marginal plate carries two separate tufts 
(“bouquets”) of spines, which somewhat resemble those of So/aster ; dorsal 
plates are present, and these bear tufts of spines; and the tufts of spines are 
* Bull. Mus. Comp. Zool., Harvard, vol. ix. No. 1, p. 17. 
VOL. XXX. PART II.. 4y 
