256 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



Surface of plates smooth or finely granulose ; the radial plates somewhat 

 elevated along their median lines • sutures distinct, but not grooved. 



Infrabasals larger than in any other known species of this genus, and 

 plainly visible from a side view, forming a shallow cup. Basals large^ a little 

 higher than wide, upper face narrowly truncated. Radials and costals of 

 similar size; the former pentagonal, the two costals hexagonal, except in 

 the posterior rays in which the second is heptagonal. Distichals to the 

 height of the sixth plate incorporated into the calyx, gradually decreasing 

 in height; the first as large as the second interbrachials, the upper one 

 resembling a good sized arm plate. Arms short, delicate, twice bifurcat- 

 ing; composed from their bases up of small triangular interlocking pieces. 

 Interbrachials : 1, 2, 2, 2, etc., rarely 1, 2, 3, 3, the lower plate heptagonal, 

 larger than the radials ; at the anal side there are three plates in the second 

 and all succeeding rows. Interdistichals nine or more, rather large. Struc- 

 ture of ventral disk and anus not known. Column circular, shghtly tapering ; 

 in the upper part of the stem the nodal joints project, but further down have 

 the same size as the others. 



Horizon and Locality. — Trenton limestone ; Montreal, Canada. 

 Tyjoes^ in the Canada Survey Museum at Ottawa. 



Archseocrinus microbasalis Billings. 

 Plate X. Figs. 2a, h, c, 



1857. Thjsanocrinus {Uhodocrinus) microbasalis — E. Billings; Geol. Rep. Canada (Rep. of Progr.) 



p. 264; also ibid., Deeade lY., p. 63, Plate 6, Fig. 2. 

 1881. Archceocrinus microbasalis — W. and Sp.; Revision Palseocr., Part 11., p. 190. 



Smaller than the preceding species, the arms proportionally longer. 

 Calyx obconical, truncated at the lower end, sides shghtly convex. Surface 

 beautifully ornamented with parallel, radiating strise, proceeding from the 

 middle of the plates to adjoining ones, and broad, rounded ridges, starting 

 from the middle of the radials and passing up the costals and distichals into 

 the free arms. 



Infrabasals minute, forming a pentagon, which rests within the basal con- 

 cavity and is covered by the column. Basals large, heptagonal, more than 

 two thirds of the plates visible from a side view, the lower end bent inward 

 and forming, together with the infrabasals, a good sized concavity, with a 

 thickened rim around the edge ; the upper faces narrowly truncated. Eadials 

 as large as the basals, distinctly angular below ; the costals somewhat smaller. 



