NO. 3 ON THE FOSSIL CRINOID FAMILY CATILLOCRINIDAE IJ 



THE ANAL TUBE 



Considering the small size of the calyx otherwise, and the extreme 

 delicacy of the arms, the anal tube of Catillocrinus is an extraordi- 

 nary structure. It is of a type occurring also in Pisocrinus, Delta- 

 crinus and probably other Cremacrinidae, and also, though rarely 

 seen, in Synbathocrinus, but here carried to the extreme, viz., a long, 

 heavy, semicylindrical appendage, reaching to (and probably beyond) 

 the ends of the arms, and thicker at the base than half the diameter 

 of the cup. It is composed of a single series of extremely heavy 

 plates, longitudinally arranged, thickest at the back (posterior side) 

 from which they are transversely curved like arm-plates, and taper 

 in a crescent to thin edges, leaving a broad, semicircular furrow at 

 the anterior. This furrow was enclosed by a flexible integument of 

 small plates, so fragile that it is not preserved intact in the fossils, 

 although I find some traces of it. Large ligament fossae on the 

 apposed faces of the plates indicate some flexibility in the tube. 

 The massive character of the tube and its component plates is seen 

 in C. tenncsseeae and C. wachsmuthi (pi. 2, figs. 14, 15 ; pi. 3, fig. 4), 

 and its extreme elongation in C. bradleyi, where it reaches a length of 

 fifteen times the height of the dorsal cup (pi. 3, figs. 14, 15), and in 

 C. carpenteri twenty and twenty-eight times (pi. 4, figs, i, 2). There 

 is little diminution in thickness after the first plate. 



In the lower part, the tube is marked externally by slight longi- 

 tudinal grooves, which are the permanent imprint of the arms when 

 closely folded against it. The lowest tube plate expands somewhat 

 towards the cup, and serves in part the purpose of a tegmen; at the 

 back it is almost as thick as the radials upon which it is superimposed, 

 and its under surface is crossed by curving furrows partly, but not 

 in full detail, corresponding to the food grooves upon the upper face 

 of the radials, for which to that extent it forms a solid roof. As 

 this lower tube plate only surmounts the upper face of so much of 

 the radial circlet as lies at the posterior side, the food grooves toward 

 the anterior must be covered by small covering pieces or perisome 

 continuous with those upon the arms. 



Now this lower tube plate, exactly at its thickest part, rests upon 

 an elevated process upon the left shoulder of the right posterior 

 radial which is usually rather narrow exteriorly, but widens consider- 

 ably inward, forming a prominent trapezoidal platform separating 

 the groups of food grooves upon the two large radials. At its outer 

 edge the tube plate is indented with a curved socket for the reception 

 of a triangular anal plate which appears between the arms at the 



