332 THE VOYAGE OF H.M.S. CHALLENGER, 
without subjacent calcareous reticulated skeleton. Paxillze small and compact. Numerous 
papulee in the interspaces. 
Marginal plates arranged in superior and inferior series, small, and covered with very 
numerous spinelets similar to those of the paxille. 
Actinal interradial areas extensive, and occupied by imbricating intermediate plates, 
arranged in isolated transverse columns, running from the adambulacral plates to the 
marginal plates: the whole area covered with a uniform epidermal layer of membrane. 
Each intermediate plate bears a single well-developed naked paxilla. 
Adambulacral plates broader than long. Adambulacral armature consisting of numer- 
ous delicate tapering spines, irregular in disposition, forming a group which occupies the 
surface of the plate, the size of the spines increasing towards the furrow margin of the 
plate. 
Mouth-plates forming a pointed mouth-angle, superficies prominent, covered with 
spines similar in form and character to the adambulacral spines, but larger. 
Ambulacral tube-feet with a well-developed terminal disk or button, devoid of spicules. 
Madreporiform body either concealed by paxillz, or large and exposed. 
No pedicellarize of any kind are present. 
Remarks.—\ have placed this genus—to a certain extent, provisionally—under the 
family Pentagonasteridx ; for although a number of its characters indicate a position 
near to such forms as Ginathaster, I consider that much might be said against its being 
ranked in this family at all, and much might be advanced in favour of its being classed 
with the Astropectinidse,—its resemblance in many points of structure to such forms as 
Leptoptychaster being very remarkable. As I do not feel at present in a position to 
clear up the doubt satisfactorily, | have placed the genus apart, though in what seems to 
me its most natural place, until a more definite opinion can be arrived at. 
Chorology of the Genus Mimaster. 
a. Geographical distribution :— 
ATLANTIC: One species between the parallels of 55° and 65° N. 
Mimaster tizardi, in the Faerée Channel. 
Pactric: One species between the parallels of 40° and 60° S. 
Mimaster cognatus, off the western coast of the southern point of 
America. 
B. Bathymetrical range: 245 to 1325 fathoms. 
Greatest range of one species: Mimaster cognatus, 245 to 1325 fathoms. 
y. Nature of the Sea-bottom: Mimaster tizardi is found on mud; DMmaster 
cognatus on Blue mud. 
