132 



THE CRINOIDEA 



is the Stem. The simplest form of columnal (after the fusion of 

 the pentameres) is circular, but with a tendency to pentagonal 

 outline. The joint-surface is radiately striated. The lumen may 

 be large and circular (Fig. XLIX. 1), or small, and circular or 

 five-rayed (Fig. XLIX. 2 and 3). The assumption of a pentagonal 

 outline is often accompanied by a restriction of the striation to 



the margin and the concentration 

 of the longitudinal ligament-fibres 

 in five bands (Figs. XLIX. 4 and 

 CXI. 7-10). The joint-surfaces 

 may become elliptical, with a 

 fulcral ridge in the long diameter 

 and ligamentar fossae on either 

 side ; in this case the long diameter 

 of one end is set at an angle to 

 that of the other end, and the stem 

 thus gets a corkscrew twist (e.g. 

 Platycrinus, Fig. XLIX. 5), and so 

 can bend in any direction. In a 

 Russian Carboniferous Platycrinoid 

 the columnals are square in section, 

 and the ridges form diagonals at 

 right angles to one another. In 

 Bourgueticrinidae, Bathycrinus, and 

 Pihizocrinus this form of joint is 

 strongly marked, and the columnals 

 are usually long and dice - box 

 shaped (Fig. XLIX. 7) ; the length 

 appears in some cases to be pro- 

 i of two at a syzygy. 

 The stem of the larval Antedon 

 (Fig. XLIX. 8, 9) has ossicles of the 

 Bourgueticrinus type and is very flexible. In two genera of 

 distinct origin Herpetocrinus allied to Heterocrinus, and Campto- 

 crinus allied to Dichocrinus the stem is rolled up round the crown 

 as shown in Fig. LIX. ; the cirri are, over the greater part of the 

 stem, confined to two rows along the sides of the ossicles and 

 directed towards the axis of the coil. In Herpetocrinus the ossicles 

 become hollowed towards the inside of the coil, and there is a 

 fulcral ridge parallel to this side ; strong (muscular ?) ligaments 

 were developed towards the outer margin. The stem could 

 uncoil and the crown be projected. The structure of the 

 columnals in Camptocrinus has not been described, but is said to 

 be similar. Wachsmuth & Springer say that such stems are 

 also found among allies of Poteriocrinus. 



The axial canal, which in recent crinoids serves to transmit the 



ne. XLIX. 



Types 



Brit. Mus. E53 ; 3, Actinocrinus, from Brit. 



Mus. 38617 ; 4, Silurian, genus indet., Brit. 



Mus. 56978, x 2'5 ; 5, Platycrinus, Brit. 



Mus. 400 ; 6, Platycrinus, showing twist 



of stem, Brit. Mus. 75906, X } ; 7, Rhizo- 



crinus, from distal region of stem, x 8; ^i^apl V.TT f 



8, Antedon sarsi, larva, articular surface of L U J L 



one of the columnals seen from the side in 



9 (after M. Sars). Much enlarged. 



