184 p. HERBERT CARPENTER. 



regarded as continuations of the ambulacral grooves of the 

 arms. 



In these Acti?iocrimdce the structure of the arms is not yet 

 known ; but in Cyathocrinus the brachial ambulacra are 

 bordered by a row of interlocking triangular plates, which 

 Wachsmuth^ compares to the '' marginal plates " in recent 

 Crinoids. They rest on an outer row of plates longitudinally 

 arranged and attached to the arm-joints. This double series 

 of alternating plates extends beneath the vault of Cyatho- 

 crinus, but the subambulacral plates appear to be absent 

 from the grooves of both vault and arms. We thus find that 

 the ambulacra of the Crinoids may be surrounded by three 

 sets of plates — (1) An upper series, present in both Cyatho- 

 crinus and Actinocrinus . (2) An under series present in 

 Actinocrinus only. (3) A lateral series, present in Cyatho- 

 crinus only (so far as we yet know). Wachsmuth has 

 already pointed out the homology of the upper series (1) with 

 the marginal plates in recent Crinoids (fig. xiv sup) , but 

 he has overlooked the fact that both the other series (2, 3) 

 are also represented, at any rate in the disc of Pentacrinus 

 caput-medusce. According to Miiller^- — 



" An den Armen und Pinnulae beschrauken sich die kalkigen Bildungen 

 auf der Ventralseite bloss auf die Saumplattchen der Ambulakralrinnen. 

 Am Kelch dagegeu sind die Ambulakralrinnen noch ausser den Saumplattchen 

 durch kalkigen Bildungen unterstiitzt. Diejenigen Pliittclien welche den 

 Rand der Ambulakralrinnen bilden, haben eine wallartige Erliohung und 

 dienen den Ambulacra Sowohl fur Einfassung als fur Stiitze der aufger- 

 ichteten Saumplattchen ; man kann sie Seitenplatten der Ambulacra 



nennen Unter der weichen Auskleiduug der Rinne 



liegen auch noch Tafelchen." 



This last set of plates (fig. xiv sub) Avas termed subambu- 

 lacral by Miiller, and there can, I think, be little doubt that 



Fig. xiv. — Diagram of the plates surrounding the ambulacra on the disc 

 of Pentacrinus. Slightly altered from J. Miiller. ad. Adambu- 

 lacral (3). an. Anambulacral. sub. Subambulacral (2). sup. 

 Superambulacral = marginal plates or "Saumplattchen (1). wv. 

 Water vessel. 



it represents the double row forming the floor of the subteg- 

 minal galleries of Acti?ioc7'inus. 



' Loc. cit., p. 122. 



2 " Bau der Echinodermen," loc, cit., p. 57. 



