236 PROCEEDINGS OF THE ACADEMY OF [1885. 



three or five. Taxocrinus Egertoni Phill. (Geol. Yorksh., PI. 3, 

 fig. 39), even has seven in one, and six in another ray. Onycho- 

 crinus very frequently has five, Ichthyocrinus generally three and 

 four in alternate rays, Taxocrinus three or four, Forbesiocrinus 

 robust us three and two, and Pycnosaccus two as a rule. Forbesio- 

 crinus Agassizi sometimes has two primary radials in one or more 

 of its rays, most frequently three, but very often four, and all are 

 articulated on a similar plan. 



The secondar}^ and higher orders of radials in the Camarata 

 rarely consist of more than two plates, sometimes, however, of 

 one, three or even four. Only Glyptocrinus, Reteocrinus and 

 allied genera sometimes have a larger number. In the typical 

 Actinocrinidae, which branch from alternate sides, the higher 

 orders consist as a rule of a single piece to each division of the 

 ray. which always at the one side supports the radial of the suc- 

 ceeding order, at the other a row of brachials. The latter, how- 

 ever, as should be expected from the term, are not free, but 

 connected laterally by suture with their fellows of alternate orders. 

 In the Ichthyocrinidae, the higher orders of radials agree in num- 

 ber and form, more or less, with the primary ones, and all are 

 similarly articulated. 



Free rays are found as a rule in the Platycrinidae ; in Euclado- 

 crinus they extend to nearhy the full length of the ray, giving off 

 alternately from every second or third plate an arm, and two at 

 the distal end. Similar rays are formed in Steganocrinus and in 

 Melocrinus; among the Rhodocrinidae in Ripidocrinus. 



The arms of the Camarata bifurcate in their free state only in 

 the genera which Zittel included under the name Glyptocrinidae, 

 in the Rhodocrinidae, and in a few Actinocrinidae, but all branch 

 at least once in the calyx. In all young specimens, as well as in 

 the earlier forms, the arms are composed of a single row of plates, 

 which gradually, embryologically and paleontologieall}', turn into 

 wedge-shaped pieces at the distal end, or even interlock, while in 

 all later genera the arms are composed of alternate joints. In 

 the Upper Silurian, the biserial arm structure predominates and 

 there is not a single species with uniserial arms in the Devonian. 

 Among the free anus there are no syzygies, but ever}' joint in 

 this group bears a pinnule, and these are frequently so closely 

 folded together, that they appear as if suturally connected. In 

 the Articulata, all arm- are composed of single joints, which in 



