18 STRUCTURE OF THE CYSTIDE.E. 



yet been ascertained. The form and distribution of these pores 

 vary greatly, but certain groups of species, closely related by other 

 characters, have them arranged after a plan common to themselves, 

 and not found in the species of other groups. Thus the genera 

 Prwioajstites (Forbes), Psendocrinites (Forbes), Apiocystites (Forbes), 

 and Lcyadocrinitcs (Conrad), belong to a group characterized by a 

 skeleton composed of a small number of plates, about twenty, which 

 are arranged in four series. All these have three pectinated rhombs, 

 one situated at the base and two near the apex. Echino-encrinites 

 (Meyer) and Glyptocystites have the same number of plates, but 

 the rhombs, although the same in general structure, are arranged 

 in a manner somewhat different from the others : Echino-encrinites 

 having two rhombs at the base and one in the upper part of the 

 body, while Ghjptocystites has from ten to thirteen rhombs ; but two 

 of these, in G. multiporus and also G. Forbesi, are situated at the 

 base of the dorsal side, in a position exactly like that of the two 

 basal rhombs of Echino-encrinites. In those genera with the skeleton 

 composed of an indefinite number of plates, the pores are circular, 

 and not clefts of considerable length, as in the others. It appears 

 therefore that good characters for classifying the species of the 

 Cystideas into groups can be derived from the form and arrangement 

 of these organs, and accordingly a classification upon this principle 

 has been proposed by Professor J. Miiller of Berlin. The outlines 

 of his system will be given hereafter. 



VI. The Column. 



The stem, stalk or column of the Cystideae is usually short and 

 tapering from the body downwards. In other respects it is the same 

 in structure as the columns of the ordinary Crinoideas. The most 

 remarkable form is the column of Lepadocrinites gebhardii (Conrad), 

 from the Lower Helderberg rocks of the United States. It differs 

 from all others known in having a large portion of the lower 

 extremity composed of a single long spindle-shaped joint. 



SECTION II. 

 ON THE AMBULACRAL ORIFICES OF THE CYSTIDE^ AND CRINOIDE^, 



If it were possible to procure a Cystidean with all the internal 

 a,nd external organs perfectly preserved, it might be easy to deter- 



