124 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



although the mouth in most of them remained closed throughout life, was 

 not morphologically distinct from that of the Crinoids of existing forms, and 

 that the so-called 'Wault " has been developed gradually from the disk. 

 The Camerata, therefore, cannot be the progenitors of recent Crinoids ; they 

 represent a side group, which in the course of Palaeozoic time departed 

 essentially from the primitive type, reaching the culmination of extravagance 

 in form and size in the Carboniferous, and becoming extinct at the close 

 of that epoch. 



B. The Anal Plates and the Anns. 



The Anal plates bear a most important part in the phylogeny of Paleo- 

 zoic Crinoids, and are also of high importance in respect to classification. 

 Some writers apply the term ^Sanal plates" indiscriminately to all interradial 

 plates of the posterior area, while others restrict it to the plates directly or 

 indirectly connected with the anus. We apply the term to the latter plates, 

 but only to such of them as take part in the dorsal cup ; the others being 

 plates of the anal tube or ventral sac. 



The anal plates in all Camerata, when present, occupy the median line of 

 the posterior area, so as to divide the interbrachial plates into two equal 

 sets ; and in rows containing an odd number they have the effect, as it were, 

 of breaking up the middle plate into two, even in cases where no anal plate 

 is inserted between the segments. The latter is the case in the Actino- 

 crinidge, in which the first interbrachial row at the posterior side always 

 consists of two plates, in place of one as in each of the others ; though all 

 have an anal plate between the radials, and an extra plate in the second 

 interbrachial row. In the Batocrinidae and Thysanocrinidse there are two 

 interbrachial pieces above the first anal, which enclose a second anal piece. 

 The Hexacrinidae have but one large anal plate resting upon the basals. 

 The Eucalyptocrinidae have no anal plate at all, the fiwe interbrachial areas 

 being perfectly symmetrical. Such is the case also in Dolatocrinus, Stereo- 

 crinus, Centrocrimcs, Allocrinus, and Patelliocrinus ; while the typical Melo- 

 crinidae have an anal plate in one or more of the upper rows. Similar 

 variations occur among the Ehodocrinidae, in which anal plates may be 

 either present or absent. The Platycrinidae have no special anal piece, but 

 the middle plate of the proximal row at the posterior side is considerably 

 enlarged, and evidently united the functions of an anal plate with those of 

 the rescular interradial. 



