154 PROCEEDINGS OP THE ACADEMY OP [1886. 



ing the ventral sac enclosed within the cal3^x. Others, such as 

 Graphiocrinus and Geriocrinus, have no azygous plate, but only 

 an anal piece ; while in Erisocrinus^ Stemmatocrinus and Encri- 

 nus no such plate is found, either within the ring of radials or 

 below. In the same degree as palaeontologically the calyx grows 

 more symmetrical, the ventral sac decreases in size, and probably 

 disappeared entirely in Encrinus, which is closely allied to the 

 Poteriocriuidse. Along with these modifications, others are going 

 on which take place in the arms. Throughout every genus, a 

 development goes on from the uuiserial to the biserial arm struc- 

 ture, a feature which has not been observed in anj^ other family 

 of the Fistulata, and which has no parallel throughout the Neo- 

 crinoidea if we exclude Encrinus, which we regard as a somewhat 

 higher developed Poteriocrinoid. Encrinus^ especially, offers in 

 its arm structure a very striking example. Among its species 

 are found arms in all stages of development, some having single 

 quadrangular joints, others two rows closely arranged. The 

 same variations are found in the arms of EupachyciHnus, Gerio- 

 crinus and Erisocrinus ; while in the earlier Poteriocrinites the 

 arms never pass beyond the interlocking stage. That the biserial 

 arm structure was developed from single cuneate joints, and not 

 only palaeontologically, but also in the growing animal, we had 

 op[)ortunity to observe in a very 3'oung Encrinus liliifortnis, in 

 which the arms, to about two-thirds their height, are composed 

 of single joints, which gradually become more cuneate, and 

 toward the tips are interlocking. This one case, we think, alone 

 would be sufficient evidence to prove that the biserial arm struc- 

 ture is the higher form, if it was not confirmed by other speci- 

 mens of this and other groups. If Encrinus were a Neocrinoid, 

 as supposed by Zittel, De Loriol and Carpenter, the Neocrinoidea 

 would begin their existence with the highest differentiated arms 

 known to us, and the arms of all other iSTeocrinoidea up to the 

 Comatulffi of the present seas, would appear to have remained 

 persistently in their larval state. This does not seem to us to 

 favor the idea, -of Encrinus being a Neocrinoid. 



Comparing Erisocrinus with Encrinus^ the only noticeable 

 difference in their fossil state is the presence of a single brachial 

 in the former, and two in the latter. To this P. H. Carpenter 

 alluded (Chall. Rep,, p. 154), admitting it to be "the only point 



