20 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 



the ponderous tube that supports the thin and numerous arms. Both 

 have the greatly thickened radial facets, which are accompanied in 

 both by the rather rare feature of perforation by dorsal canals (see 

 various figures on plates 2 and 5). Their respective families both 

 started in the Devonian and culminated in the Lower Carboniferous 

 of America or in the Permian of Timor. If we take Pisocrinus as 

 the beginning of this line in the Silurian, then both ran a parallel 

 course with the Cremacrinidae, except that the latter started first in 

 the Ordovician, ended a little earlier in the Keokuk, and have not 

 thus far been reported from Timor. 



The description of the anal tube of Synbatliocrinus given by 

 Wachsmuth and Springer in Revision of the Palaecrinoidea, pt. 3, 

 p. 167, pi. 4, fig. II, as being composed of five longitudinal rows of 

 plates, will have to be corrected. We have it now preserved in several 

 specimens, and its structure is uniformly as shown in figs, i and 4 of 

 plate 5, namely, a single longitudinal series of strong crescentic plates 

 at the posterior, having the curve completed by an integument of 

 small plates, substantially as in Catillocrinus. 



THE ARMS 



The arms are the most delicate in proportion to their length of any 

 known in the fossil crinoids. They are unbranched, very long, ex- 

 tending nearly to the end of the tube, around which they are closely 

 packed and supported when at rest in shallow grooves, abutting all 

 around in a continuous circlet except for a short distance at the 

 posterior side, where they are separated by the anal and tube plates 

 above mentioned. They differ in size in the different species, those 

 of C. tcnnesseeae being the thickest, about 1.33 mm. in width, while 

 in C. bradleyi the arms are less than .50 mm. wide, being thus not 

 only relatively, but actually, the thinnest of all. The fact that several 

 arms, up to as many as 31 on a single plate, are borne directly upon 

 some of the radials, differentiates this form and its congeners from 

 the other crinoids generally, although this structure was fore- 

 shadowed in Calycanthocrinus of the Lower Devonian. 



Considerable importance has been attached to the number of arms, 

 especially by Jaekel, who has taken the increasing number as a con- 

 trolling character in the progressive developmental series by which 

 he traces the evolution of this type, conformably to the geological 

 succession, from Pisocrinus of the Silurian with 5 arms, Calycantho- 

 crinus and Mycocrinus of the Devonian with 9 to 17 arms, to the 

 successive Lower Carboniferous species of Catillocrinus from C. 



