0)1 British Palceozoic Crinoids. 295 



recent Crinoid. We see no reason to believe that the plates 

 bordering these ambulacra were not movable during life, like 

 the similar ones on the ambulacra of the arms, so that the 

 food-grooves were completely open to the exterior. In fact 

 Wachsmuth * admits that " this might possibly have been 

 the case in Cyathocrinuf^ iowensis ; but I even doubt it here, 

 as the corresponding plates in other closely-related species, 

 though arranged upon the same fundamental plan, present 

 rather an aspect of true vault-pieces." Whatever may have 

 been the case in the Cyathocrinida3, we believe that the ventral 

 disk of the Ichthyocrinidaj, which was composed of " a more 

 or less soft or scaly integument yielding to motion in the body 

 and arms," was essentially like that of a recent Crinoid with 

 movable plates bordering the ambulacra and an irregular 

 pavement in the interradial areas. We cannot therefore 

 regard all the Palseocrinoids as having been without external 

 food-grooves, as is supposed by Wachsmuth and Springer ; 

 and we think it also quite possible that the apical dome-plates 

 of the Cyathocrinidas and Ichthyocrinidaj were movable during 

 life, so that the mouth was open to the exterior. Hence we 

 do not attach quite so much importance to these two charac- 

 ters as do Wachsmuth and Springer. But we regard the 

 j)resence of the apical dome-plates or of a true vault, and not 

 the condition of the mouth and food-grooves, as an important 

 distinction between the older and the younger Crinoids. It 

 is certainly a more constant one than the absence of axial 

 canals in the radials, though not altogether universal. 



Probably the most constant difference between the Palaeo- 

 zoic and the younger Crinoids is one to which we do not think 

 attention has yet been drawn. In almost all the Mesozoic 

 and recent Crinoids the calyx is perfectly regular and sym- 

 metrical all round f. There are five equal and similar basals, 

 upon which rest five equal and similar radials ; and each of 

 these is in close lateral union with its immediate neighbours 

 without the intervention of any interradial pieces at all J. 

 Should there be any interradials in the calyx, as between 

 adjacent second or third radials, they are not limited to any 

 special side of the calyx, but are equally distributed all round 

 it, as in Guettardicrinus and Apiocrinus. Lastly, if the rays 

 divide, it is always the third radial that is the axillary joint. 



* Palseozoic Crinoids, p. 184. 



t The distortion of the calyx in the Eugeniacrinidse may be left out 

 of consideration for the present. 



X The partial freedom of the first radials from one another in Bathy- 

 crinu/i and Peniacrhms suhani/ulan's does not affect the question under 

 discussion, owing to the absence of interradials in these genera. 



