rLATYCEiyiD^. DICIIOCRINUS. 45 



Geiiiis 11. DICHOCRINUS. (Munsler) 



Defiiiilion. — Dorso-central plate hexagonal and bipartite; peiisonilc plates six, five 

 of wliich bear the rays. Column and base of attachment unknown. 



We had long suspected that a crinoid intermediate in character between the so called 

 Plalycrini of the Devonian strata and the true Platycrini of the mountain limestone would 

 be discovered, and accordingly on a careful examination of numerous specimens we have 

 met with sufficient evidence to prove that such intermediate form does occur in the 

 carboniferous limestone. This interesting fact exhibits in a very striking manner the 

 beautiful gradations in organic structure from Crinoids with a tripartite hexagonal dorso- 

 central plate, on which rest six perisomic plates, to a bipartite dorso-central plate, to 

 which six perisomic plates adhere, and finally to the true Platycrinus with its undivided 

 pentagonal dorso-central plate bearing but five perisomic plates. 



We had made these observations before we were acquainted with the fact, that Count 

 Munster had already arrived at the same conclusion. 



Miller in his work on the Crinoidea, at page 114. has given the figure of a bipartite 

 dorso-central hexagonal plate, but which he considered a monstrosity. With this idea he 

 made the following observation. "I consider this as a monstrous variety oi Plalycrlniles 

 lavis, analogus to those irregularities of nature which occasionally render the column of 

 the Pentacrini tetragonal and hexagonal," &c. 



The dorso-central plate here alluded to by Miller, may be referred to the genus 

 Dichocrinus, and it will probably be found that all those crinoids with six perisomic 

 plates, which occur in the carboniferous limestone, have a bipartite dorso-central plate 

 and are consequently not Platycrini but Dichocrini. 



The six plated specimens heretofore considered as the P. elonyatus by Mr. Phillips, 

 and referred to in a former number of our monograph as such, will properly appertain to 

 this new genus of Count Munster. 



Species 1. Dichocrinus radiatus. {Minister.) 

 Pl.V. fiy. 5. a, b, c, d. 



Definition. — Dorso-central plate bipartite, hexagonal and moderately conical; perisomic 

 plates six, five of which bear the rays. All the plates are smooth. Column and rays 



unknown. 



