212 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Thus the radicular cirri represent a structure peculiar to the distal end of the 

 column, and have nothing whatever to do with the other cirri, which, in the recent 

 forms, are never known to occur beyond the first stem syzygy. 



In the comatulids the single syzygy in the stem occurs between the centrodorsal 

 and the columnal next beneath it (fig. 553, pi. 5); in the pentacrinites each nodal 

 is united to the columnal just beneath it by a syzygy, which in structure and in 

 location is the exact counterpart of the single comatulid stem syzygy (figs. 127, 

 p. 197, and 143, p. 205). In the bourgueticrinoid type of column any two of 

 the columnals may be united by syzygy, these double columnals usually being rare 

 in the distal portion of the stem and increasing in frequency near the crown (fig. 129, 

 p. 201; a stem syzygy is seen at the letter 6). While in the comatulids and in the 

 pentacrinites the columnal just above a syzygy always gives rise to cirri, which, 

 though usually five in number (fig. 127, p. 197), may be as few as two or even 

 one, or may go to the other extreme and be as many as 80 or more, in the bourgueti- 

 crinoid type of column this does not occur, the epizygal (if this term may be used in 

 this connection) being in no way different from the hypozygal, the two being closely 

 united, with the line of union slightly everted. 



The syzygies in the stems of the stalked crinoids are not in any way homologous 

 with those in the arms ; though they are structurally and mechanically identical, this 

 identity means nothing more than formation under ontogenetically similar con- 

 ditions of structures with radically different phylogenetic antecedents. 



The syzygies in the arms occur between two ossicles which, in the transformation 

 from a biserial to a uniserial condition, have not quite succeeded in fusing into a 

 single ossicle, and, on the other hand, have not retained their individuality. The 

 syzygial pairs of the arms are intermediate in character between the ossicles of the 

 division series and first two brachials of the free undivided arm, each of which is 

 primarily a double structure, and the outer brachials, all primarily single structures. 



All recent and mesozoic crinoids possess a proximale or a strictly homologous 

 structure, typically single and attached permanently to the calyx, as in Millericrinus, 

 Bourgufticrinus, Phrynocrinus (fig. 2, p. 61), Thiollericrinus and the comatulids, 

 but sometimes multiple, occurring all together just under the calyx, as in Apiocrinus, 

 or at regular intervals throughout the column, as in the pentacrinites (fig. 127, p. 197), 

 or at frequent intervals in the proximal portion of the column and becoming less 

 common distally, as in Proisocrinus (fig. 128, p. 199), Rhizocrinus, Batliycrinus, 

 MonacTiocrinus, and Democrinus. 



The proximale primarily denotes the maturity of the column and the comple- 

 tion of stem growth, and is therefore quite analogous to the large lip developed in 

 the Helicidse and in other gastropods. It is normally the last columnal to be 

 formed and, as no further columnal formation occurs, it becomes intimately attached 

 to the calyx, fusing with the infrabasals and forming to all intents and purposes a 

 dorsal calyx plate. The proximale probably secondarily represents the original 

 central calyx plate from which the stem was developed by a more or less complex 

 process of reduplication. 



Welded to the dorsal surface of the calyx by a union exactly similar to that 

 between the basals and the radials, by a close suture which to all intents and purposes 



