MORPHOLOGICAL PART. 59 



h and i, respectively, are united (Fig 13). As this structure is apparently 

 constant in the latter group, we regard it as of some importance as regards 

 classification. 



The introduction of the anal plate did not affect the basals of dicyclic 

 Crinoids in the same manner as in the monocyclic. While in the latter, 

 when the plate is represented, the orientation of the basals is slightly dis- 

 turbed, in the dicyclic forms it remains unaltered. The anal plate of the 

 latter rests invariably upon the truncated upper face of the posterior basal 

 (see Figs. 14 to 18) ; while in monocyclic Crinoids it is supported by the 

 basals a and e (Figs. 10 and 12), or occasionally by a and x (Fig. 8). 



The infrabasals are completely anchylosed in the Cupressocrinidse, Gas- 

 terocomidse, and in Stemmatocrinus, where they form an undivided disk, which 

 Carpenter and others have regarded as representing the top stem joint, as in 

 the case of the Apiocrinidse and Comatulse. Agasskocrinus, in its pedun- 

 culate younger state, had five well defined infrabasals ; but after losing its 

 stem, the suture lines became gradually obhterated by limestone deposit 

 upon the surface. The same is the case with the basals of the monocyclic 

 Edriocrimis, 1 



Mr. Bather discriminates between Dicyclica, Pfceudomonocyclica, and 

 Monocyclica vera.* To the Pseudomonocyclica he refers those forms in 

 which infrabasals are obsolete in the adult, but were represented in early life. 

 They embrace most of the Mesozoic and recent Crinoids, and may be sub- 

 divided into two classes : (1) forms in which the infrabasals gradually 

 become anchylosed with the top stem joint, and (2) those in which they 

 were resorbed in the adult. In the former, which among other groups 

 include the Apiocrinidae and Comatulse, the new stem joints are formed below 

 the centro-dorsal ; while in the latter, which are typified by the Pentacri- 

 nidae, the top stem joint is the youngest joint of the stem. We shall 

 presently show that both these forms, although the infrabasals may have 

 disappeared, still retain the characteristics of dicyclic Crinoids. 



Several years ago we discovered t that among the Palaeocrinoidea there 

 is a regular alternation in the arrangement of the successive parts below the 

 radials, and that the orientation of the stem is essentially different among 

 monocyclic and dicyclic forms. We found that the salient angles of the stem 

 itself, and the projections of the axial canal, are reversed in the two groups, 



* Ann. and Mag. of Nat. Hist, (sixth series), Vol. Y., April, 1890, p. 316. 



t Revision, Part III, Section I, p. 7 (Proceed. Pliila. Acad., 1885, p. 229), with a most unfortunate 

 transposition of terms, which was corrected in the appendix. Also 1888, Proceed. Phila. Acad. p. 351. 



