1885.] NATURAL SCIENCES OF PHILADELPHIA. 283 



abty less substantial, and perhaps in some of them altogether 

 membranous. We never observed anything like plates in the 

 Actinocrinites, except in Physetocrinus, in which they are well 

 developed. Of this genus we lately obtained a specimen which 

 proved that the small tubercles along the ventral surface, figured 

 by us in Rev. ii, PI. 19, fig. . r >, are not, as we then supposed, 

 openings through the " vault," but impressions of the open 

 spaces between the anambulacral plates. In Actinocrinus and 

 allied genera we occasionally find little pillars or nodes along the 

 inner floor, which sometimes suspend fragmentary plates, or 

 portions of a filmy substance, and evidently are parts of the 

 perisome. We found similar pillars along the floor of Glypto- 

 crinus rarnulosus (PI. 9, fig. 2) underneath the interradial areas, 

 but not beneath the plates overlying the ambulacra. The latter 

 plates are folded as in Physetocrinus, and formed into natural 

 grooves, which evidently harbored the ambulacral tubes. 



The ambulacral tubes of the Actinocrinidae rest upon the peri- 

 some, but rarely enter the plates of the vault, and do not become 

 exposed until they enter the free arms. In the Platycrinidae the 

 structure is essentiall}' the same, but the covering pieces frequent^ 

 enter the calyx at — or close to — the proximals, and in this case 

 often take the form of vault plates. In the young Crinoid, 

 according to our interpretation, the ambulacral tubes were 

 attached to, and rested primarily within the grooves of the lower 

 arm joints, from which they were gradually lifted out when these 

 became incorporated with the calyx and transformed into radials. 

 It seems to us that, while this was going on, the radial regions 

 of the vault were raised by the ambulacra, thereby producing 

 elevations or folds along the vault of Glyptocrinus and Physeto- 

 crinus ; while in Platycrinus the ambulacra in many cases pene- 

 trated the test. 



The tubes are composed of four rows of plates, alternately 

 arranged, of which two constitute the floor, the two others the 

 upper side. The upper ones are the covering pieces, but we are 

 not certain whether those at the floor are side pieces or form a 

 sort of suliambulacral plates. The covering plates where they 

 entered the vault were suturally connected, but on entering the 

 arms became movable. Side pieces have never been observed in 

 the Camarata, but covering plates are found occasionally both in 

 arms and pinnules, and were probabby present in all of them. 



