264 PROCEEDINGS OF THE ACADEMY OF [1885. 



part of the tube. They do not all, however, have the tube well 

 developed, and in some of them it is altogether unrepresented. 

 Haplocrinus has a simple anal opening, and herein deviates 

 from most of the other genera of this group. The Hybocrinidre 

 and Cyathocrinus alutaceus have only a short protuberance, com- 

 posed of small plates. 



Tlic simplest tube is found in Catillocrinus and Calceocrinus 

 (PI. 5, figs. 15, 16), in which it consists of a single row of very 

 large solid plates, transversely curved like an arm-joint, with a 

 semicircular groove along the ventral side. This groove, which 

 extends from the base of the tube to its distal end, is open in all 

 our specimens. A somewhat similar tube occurs in Symba/In). 

 crinus, in which the proximal plates at the posterior side are 

 considerably thicker than those upon the other sides. 



More important from a morphological standpoint, is the ventral 

 tube of the Heterocrinidse, Anomalocrinida- and some Silurian 

 C3"athocrinoid genera in which the anal piece, as in Catillocrinus, 

 is succeeded by a row of heavy curved plates, which on the dorsal 

 side pass up to the end of the tube. These plates are bordered 

 laterally by several rows of delicate pieces, pierced by pores or 

 slits along their sides, the whole forming a sac-like appendage. 

 It is very evident that this row of dorsal plates is identical with 

 that of Catillocrinus and Calceocrinus, and also that the ventral 

 side of the tube in the latter two genera was closed by plates in 

 :i similar way. 



A still higher form is represented l>\ the later Cyathocrinidse, 

 which have no such row of dorsal plates, the entire sac being com- 

 posed of delicate pieces. Most of these are perforated with pores, 

 with the exception of the proximal rows of plates dorsally, which 

 are solid, and also frequently those crowning the distal end; 

 while those facing the ventral side are more or less perforated. 

 Among the earlier Poteriocrinida-, the sac is large, either cylin- 

 drical, club-shaped, conical or balloon-shaped, and it often extends 

 beyond the tips of the arms. In the later Poteriocrinidae, how- 

 ever, the sac dwindles down to a short cone, even in the asym- 

 metrical EujMchycrirnis, and it lias apparently no pores, at least 

 not dorsally. 



In most of the Camarata the anal plate is placed between the 

 first radials, and occupies the lower portion of the dorsal cup. 

 in the Rhodocrinkhe, in which the first interradials alternate with 



