156 SYNOPTIC COLLECTION. 



The mouth and five ambulacra are without covering 

 plates. The arms of the very young Encrinus are at first 

 made of single sections, that is, they are uniserial, but 

 afterwards they become biserial 1 a proof that the uni- 

 serial condition is the more primitive. 



In the Crinoids there were no tubes or hydrospires, but 

 respiration took place through pores between the plates 

 of the oral surface. 



Marsupites (No. 274) is an instructive form. In youth 

 it is attached by a stem, but later it breaks away, and the 

 rounded posterior part of the body usually shows no scar. 

 Underbasals, basals, and radials are all present ; these 

 are thin and flexible. 2 The ventral disc is not preserved 

 in any of the specimens. The arms are uniserial, but are 

 usually broken off. 



The second series of Crinoids is more specialized, 

 speaking generally, than the first. 



Platycrinus (No. 275, P. hemisphericus\ PI. 276, P. tri- 

 gintidactylus Aus.) has a body made of basals and radinls, 

 the former of which are unequal. From the ventral side 

 rises a large anal tube (see PI. 276). The arms are free, 

 they fork a few times, and are well supplied with pinnules. 

 The ambulacra are concealed by covering plates. 



Actinocrinus (No. 277) has a small body without 

 underbasals, and the arms are attached near its middle. 

 The basals are reduced to three in this genus. The 

 specimen (No. 277 a) has eight arms, and the pinnules 

 of one are fairly well preserved. The food was caught 

 by the pinnules and carried down to the base of the arms 

 where it passed through the covered tunnels of the ambu- 

 lacra to the mouth. The convergence of these ambula- 

 cral grooves a little to one side of the center is seen in 

 the internal casts of Actinocrinus (No. 277 b-d) . Here 



1 Wachsmuth and Springer, Proc. Acad. Nat. Sci. Phila., 1886, p. 

 230. 



2 Bather, Proc. Zool. Soc. London, 1895, p. 996. 



