254 THE CEINOIDEA CAMEEATA OF NOETH AMEEICA. 



obscure, flattened ridges, which grow more conspicuous on approaching 

 the arms. Infrabasals small, rarely extending beyond the column, and 

 resting, as a rule, within a concavity formed by the lower part of the 

 basals. Basals heptagonal, their upper faces truncated. Radials in three 

 of the rays pentagonal, in the two posterior ones frequently hexagonal. 

 When there are no palmars the three to six lower distichals take part in the 

 calyx, and the succeeding ones are free arm plates. Arms short, slender, 

 branching ; composed of two rows of cuneiform pieces, alternately arranged 

 and interlocking. Interradial areas constructed of numerous plates; the 

 plate between the radials, which is much larger than the others, is followed 

 at the regular sides by two interbrachials in the second row, at the anal side 

 by three, but there is no median ridge or continuous row of anal plates. 

 The upper interbrachials connect imperceptibly with the plates of the disk. 

 Interdistichals always represented. Ventral disk, as observed in A. desideratus, 

 composed of small irregular pieces, which close over the ambulacra, except 

 near the outer margin of the integument, where their covering pieces are 

 exposed. Column round, the edges of the nodal joints largely projecting 

 over the internodal ones ; axial canal large, pentalobate. 



Distribution. — Probably restricted to the Trenton group of America. 



Type of the genus : Archceocrinus lacimosus (Billings). 



Itemarhs. — Archmocrinus has close affinities with RJiodocrinus, and it is 

 somewhat difficult to point out the structural differences. The calyx of the 

 former is relatively larger, the arms shorter, and it has but two interbrach- 

 ials in the second row; while Wiodocrinus^ as a rule, has three plates in the 

 second and all succeeding rows ; and the anal interradius very often has the 

 same arrangement of plates as the others. 



Glyptocrinus marginatus Billings, which in 1881 we placed under Archceo- 

 criniis^ proves to be a monocyclic form, and will have to be referred back to 

 Glyptocrinus as a somewhat aberrant type. Whether Rhodocrinus asperatiis 

 Billings belongs here, cannot be ascertained from the imperfect state of the 

 specimen. Lyriocriniis sculptus S. A. Miller {Archceocrinus scidptus W. & Sp.) 

 is a synonym of Rhodocrinus vesperalis White, which has been placed under 

 the genus Diaholocrinus. 



