104 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



but covering pieces, which gradually in geological time changed their 

 character. 



The " radial dome plates " of the Actionocrinidge and allied forms are 

 generally larger than any of the surrounding plates, often nodose, and some- 

 times extended into long spines. They are not followed immediately by 

 covering pieces, as already stated, and are placed at some distance from 

 the orals, — occupying in the simpler forms with but two arms to the ray 

 almost the outer margins of the tegmen {Agaricocrinus), — directly over the 

 point at which the bifurcation of the ambulacra takes place. When there are 

 four arms to the ray, they are removed relatively further inward, and are 

 followed by two similar but smaller plates of higher rank. But when there 

 are three arms to the ray, there is only one such plate, which is directed to 

 the side where the bifurcation is, the opposite side of the plate being followed 

 by the regular covering pieces of the arms. 



From this structure we may infer that the so-called radial dome plates 

 with subtegminal ambulacra are axillaries, and if they represent, as we have 

 reason to believe, modified covering pieces, that they are the plates from 

 which the ambulacra bifurcate. In this view it is quite suggestive that the 

 axillary plates of the ambulacra are frequently protuberant. In Eucladocri- 

 mis miUehrachiatus (Plate LXXIII. Fig. I), they are all along the main arms 

 strongly nodose, and if the ambulacra of this species had been covered by 

 other plates, the tips of the axillaries naturally would project above them 

 and be exposed upon the disk. In this way the radial dome plates may have 

 originated, so that afterwards the upper portions developed to larger size, and 

 finally become independent plates. This explanation seems to us most prob- 

 able, and it was favorably received by Carpenter. 



The Palaeontological evidence indicates that in the earlier Camerata, as in 

 the young specimen at some time, the ambulacra were exposed upon the 

 disk. In most of the Silurian forms they took part in the tegmen, and their 

 covering plates, as a rule, were more regular in their arrangement than in 

 those of later epochs. In the Carboniferous, with the exception of the 

 Platycrinidae, Hexacrinidae and Acrocrinidae, the ambulacra are almost 

 exclusively subtegminal, and the whole disk assumes that extravagant 

 form which led at one time to the belief that it represented an entirely 

 different structure. 



