< m.wioi i;i\i g i > i • > m i i ». i:. 39 



Carpenter is now of opinion thai the ventral plates of the Ci srata are 



in their essential characters nothing but more <>r less higblj developed 

 perisoraic plates. He has long believed this aboul Glyptocrinus, Reteo- 

 crinus, and similar forms. Recent work on Encrinus, Apiocrinus, and Cala- 

 mocrinus lias led me gradually to disbelieve in any vaull as a system ol 

 plates distinct from a disk. 'W 1 1 » • 1 1 a distinct line of covering plates mark- 

 ing the ambulacra is nol traceable, as in E. Briareus and A. Roiyssianus, 

 this is not line to the ambulacra being subtegminal, as frequently asserted. 



The real cause. I am com in I. is not this, bu1 that the covering plates 



are not clearly differentiated from the other plates of the dish. 



Carpenter also says that the vault of a Plat \ crinoid (('hall. Hep., p. L80) 

 corresponds collectively to the oral, interradial, ambulacra!, and anambula- 

 cral plates of Neocrinoids, ami that (('hall. Rep., p. 171) the mi called vault 

 of the genus Marsupiocrinus* is really the strongly plated ventral perisome ; 

 and furthermore, that he can develop the heavy vault of the Carboniferous 

 species from the disk of the Silurian forms by a gradual course of pahoonto- 

 logical development, although at the same time other lines of development 

 may lie going on; and finally, that the radial dome plates of the Actino- 

 crinidae are to he regarded as extravagantly developed covering plates, 

 and that the term calyx interradials should be given up. 



Carpenter and Wachsmuth and Springer are now fully prepared to admit 

 that all the plates between the rays are parts of one and the same system, 

 whether they be massive, like the first interradials of Guettardicrinus, or 

 delicate, as in the recent Pentacrinidae. 



As has been stated by Carpenter, in the recent Crinoids thus far described, 

 they all with the exception of Thaumatocrinus have the first radials united 

 so as to form a complete ring all round the calyx; hut in that genus the 

 primary radials are separated by an interradial resting upon the basals 

 below. These calyx interradials. as they have been called, are vety gener- 

 ally present in Palreocrinoids. and help to increase the size of the cup. 



In Belemnocrinus an anal plate separates two of the radials. In Hexa- 

 crinus the same is the case. In the Mesozoie Apiocrinidae " there are 



* Scbluter gives excellent figures and a good analysis of Uintacrinus (Zeitschr. d. Deutsch Geol. 

 Ges., 1S73), showing remarkably well the gradual passage of the so called large calyx interradials into 

 comparatively small actinal plates. The same structure is repeated in the angle of each pair of arms 

 in the interpalmar spaces, the first large plate of the latter series resting upon the first two distichals, 

 exactly as the first plate of the interradi.il area rests upon adjoining first radials. The centra] plate, 

 without any perforation, is surrounded by a circle of five basals and five adjoining radials. with inter- 

 radials fully as large as the radials. 



