64 PROCEEDINGS OP THE ACADEMY OP [1886. 



REVISION OF THE PAL^OCRINOIDEA. 

 BY CHARLES WACHSMUTH AND FRANK SPRINGER, 



Part III Section II. 

 SuBOKDEK ARTICULATA. 



The Articulata include the group formerly defined by us under 

 the family name Ichth3'ocrinid{e, with the addition of Grotalo- 

 crinus and Enallocrinus, which possess in a remarkable degree 

 some of the most characteristic features of the group. We have 

 elsewhere shown that our former definition of the structure of 

 the ventral surface in the Ichthj'ocriuidae was faultj'^ in the use of 

 the word " soft," in which we did not have in mind the idea of 

 membranous as opposed to calcareous, or of disk as opposed to 

 vault, but simply employed the word to express more stronglj^ the 

 notion that the vault was not rigid. We maintain, however, that 

 the outer test of the ventral side in this group was a continuous 

 integument, composed of calcareous plates, united by ligament 

 and not by a close suture, and that by reason of this structure and 

 the articulation among the plates of the dorsal side it must have 

 been pliant or flexible. The exact nature of this integument we 

 do not know. The plates may have been arranged in various 

 waj^s ; they may even have been imbricated in some types like 

 the interambulacral plates in some Periechocrinidae, and even in 

 some of the true Echini these are points we may perhaps never 

 be able to settle. That there was an inner integument roofed in 

 and covered by the flexible vault we have mentioned, and that it 

 contained the summit plates and " covering pieces " we know to 

 be true in the Crotalocrinidae, and we think it altogether probable 

 that the general plan of the ventral structure for the Articulata 

 generally is expressed in that of Grotalocrinus. 



The eflfect of the patelloid plates, observed in Forhesiocrinus^ 

 in permitting mobility in the whole skeleton, has been heretofore 

 mentioned. The suggestions we then made are confirmed by the 

 discovery of the remarkable articulation not onl}'^ among the 

 radials themselves, but also between the radials and interradials 

 (PL 6, figs, 3-5). This articulate structure, and the consequent 

 mobility in the test, and flexibility^ of the vault, we consider 



