88 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



base, tapering at an angle of a little less than 45. The five oral 

 plates are subpentagonal, about half a line in width. The first 

 adambulacral plates of each pair of adjacent rays are in contact 

 with each other outside of the oral plates, and not completely sep- 

 arated as they are in P. niagarensis. There are six or seven adam- 

 bulacral plates on each side of the ambulacral groove in each ray, 

 and they gradually decrease in size from the oral plate outward to 

 the point of the ray. The width of the ambulacral groove is equal 

 to one-third the width of the ray and consequently the adambu- 

 lacral rows of plates are also each equal to one-third the whole width 

 of the ray. In each groove there are two rows of small and appar- 

 ently nearly square ambulacral plates, 12 or 14 in each row, and they 

 seem to be continued round on the inner margin of the oral plates; 

 the mouth is about one line wide." 



Emended description. Measurements: R = 6 mm., r = 2.5 mm., 

 R = 2.4r. Width of ray at base 3 mm. 



Rays short, stout, tapering rapidly. Disk comparatively large, 

 and without interbrachial arcs. 



Abactinal area unknown. 



Inframarginal plates at the base of the rays very large and thick, 

 highly convex, duninishing rapidly distally, usually quadrangular 

 in form and seven on each side of a ray. 



Just within each axil, inside of the basal inframarginals, there is 

 a single, large, very convex, pentagonal, axillary interbrachial plate. 



Adambulacral plates gradually sinking below the inframarginals 

 proximally, convex and quadrangular in form. There are 13 plates 

 in each column adjoining the very narrow ambulacral furrow, or 

 nearly two plates to each inframarginal ossicle. Two of these 

 plates (oral armature) are situated in front of each axillary inter- 

 brachial, uniting the adambulacral columns of adjoining rays. 



Ambulacral plates unknown. 



Locality and formation. The type-specimen, a natural mold, was 

 found by Rev. D. Honeyman in the Lower Arisaig of the Siluric 

 ( = Clinton) rocks at Arisaig, Nova Scotia. The holotype is in 

 Redpath Museum of McGill University, Montreal, Canada, and the 

 illustration is taken from a gutta-percha squeeze kindly made for 

 the United States National Museum by Sir William Dawson. 



Remarks. The general actinal appearance of this little asterid 

 reminds one much of Palseaster and Hudsonaster. It differs, however, 

 at once in a little detail of considerable importance, in that each 

 axillary area has three plates, while in Palseaster and Hudsonaster 

 there is but one. In other words, the large axillary plate of those 

 genera has in M. (?) parviusculus been pushed orally and is here an 

 interbrachial plate, while the axils of the rays are occupied by two 

 basal inframarginal plates. For further remarks on this species see 

 M. granti (Spencer). 



