332 THE VOYAGE OF H.M.S. CHALLENGER. 



without subjacent calcareous reticulated skeleton. Paxillse small and compact. Numerous 

 papulae in the interspaces. 



Marginal plates arranged in superior and inferior series, small, and covered with very 

 numerous spinelets similar to those of the paxillae. 



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 paxillse, or large and exposed. 



No pedicellarise of any kind are present. 



Remarks. — I have placed this genus — to a certain extent, provisionally — under the 

 family Pentagonasteridse ; for although a number of its characters indicate a position 

 near to such forms as Gnathaster, 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 Astropectinidae, — its resemblance in many points of structure to such forms as 

 Leptopty 'chaster being very remarkable. As I do not feel at present in a position to 

 clear up the doubt satisfactorily, I 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 c N. 



Mimaster tizardi, in the Faeroe Channel. 

 Pacific : One species between the parallels of 40° and 60° S. 



Mimaster cognatus, off the western coast of the southern point of 

 America. 



/3. Bathymetrical range: 245 to 1325 fathoms. 



Greatest range of one species : Mimaster cognatus, 245 to 1325 fathoms. 



j. Nature of the Sea-bottom: Mimaster tizardi is found on mud; Mimaster 

 cognatus on Blue mud. 



