360 PROCEEDINGS OF THE ACADEMY OF [1881. 



Albany Inst., iv, p. 202) have by no means cleared up the difli- 

 cnlties attending their separation. 



According to Hall, the principal distinction should be found in 

 the rudimentary or imperfect condition of the underbasals in 

 Glyx)tocrinus^ as opposed to the moderately large size of the same 

 pieces in Glyptaster, and in that the rays of the former were twice 

 subdivided in the body, with simple undivided arms, while the 

 latter had but a single division in the bod3^ A comparison of all 

 the species that have been referred to Glyptocrinus, shows con- 

 clusively that these supposed distinctions are not borne out by 

 the facts. Hall's so-called Glyj)tocr. O^Nealli has well defined 

 underlmsals, and the second branching of the ray takes place in 

 the free arms, while the rays in Glyptocr. nobilis divide at least 

 three times in the body, and on the other hand in G. Dyeri Meek, 

 the second bifurcation takes place in the arms. We have placed 

 G. O^Nealli Hall, G. Richardsoni and G. gracilis Wetherby, 

 G. Baeri Meek, and G. cognatus Miller, under Reteocrinus 

 Billings, and should have proposed G. nobilis the type of a new 

 genus, if we had before us good specimens instead of figures of 

 casts in which neither basal nor anal plates are shown. The high 

 dome, the anal tube, the large number of arms, and its occurrence 

 in a different geological horizon suggests very strongly to our mind 

 its generic distinctness. 



Glyptocrinus is one of the oldest forms of the Rhodocrinidse, 

 and is the forerunner and type of a little group of Silurian Crinoids, 

 which are readily recognized by their obconical form ; the promi- 

 nent rounded ridges that follow the radial series of the calyx, 

 '.coking like recumbent arms ; the beautifully striated surface, and 

 the character of the arm structure. The arms in all of them 

 rise from the edge of the vault, forming uninterrupted prolonga- 

 tions of the elevated ridges of the calyx, and the radial plates pass 

 into arm plates so gradually, that it becomes almost impossible to 

 discern where the cal^^x terminates and the arms begin. Zittel 

 has arranged these genera into a distinct family and we should 

 follow his example if we had not discovered that the same charac- 

 ters exist among the Actinocrinidse, and indeed — what is more 

 significant — among genera with five, four and three basals, but 

 only in Silurian types, thus indicating that the characters above 

 noted represent probably a younger stage of familj^ development. 



Glyptocrinus differs from Glyptaster in having rudimentary 



