44 CALAMOCEINUS DIOMED.E. 



to that of the recent Crinoids can only be traced by a careful comparison 

 with the oral plates of some of the Idunata and with the young stages 

 of Comatula. 



In the latter we have a subtegminal mouth and interradial orals, and 

 we have in the former interradial orals and both subtegminal and exposed 

 ambulaeral furrows, where the ambulacral covering plates and side plates 

 do not reach the interradials, whether they be orals or other interradial 

 plates have encroached upon this space and covered the central part of the 

 disk, exactly as we find in Spatangoids with disconnected apical systems, 

 the intervening space filled by the junction of opposite interambulacra 

 to occupy the space of plates belonging to the ambulacral and apical 

 systems; but here they are pushed aside, while with some Ciinoids they 

 are covered. 



There is no line of demarcation between actinal and abactinal in the 

 perisomic plates, and none in the Cj'stideans; and there are all possible 

 stages between Crinoids in which the contrast is sharply defined, and 

 those in which it is indistinct and impossible to di\aw the line. And this 

 encroachment of the interradials can nowhere be better observed than in 

 such a vault structure as Cj^athocrinus malvaceus, in which they are few 

 in number and simulate the arrangement of the oral plates. 



The above shows the great changes which have taken place in Wachs- 

 muth's earlier views of the homologies of the vault of the pala?ozoic Crinoids 

 and of the disk of Neocrinoids, which at one time he considered it impossible 

 could in any way be homologized. 



Carpenter in the Challenger Report (p. 185), after an extended discussion 

 of the structure of the vault in palaeozoic and other Crinoids, sums up his 

 conclusions regarding the homologies of the vault of the pala30zoic and 

 Neocrinoids as follows : " In Cyathocrinus, Platycrinus, Glyptocrinus, Reteo- 

 crinus, and Xenocrinus, as well as in the Ichthyocrinidte, the resemblance 

 to the Pentacrinidfe, Apiocrinidce, and Comatulidte is such as to leave no 

 reasonable doubt that the so called vault of the Paliieocrinoids is homologous 

 with the ventral sui'face of the body in the Neocrinoids. . . . Yet the mouth 

 may have been subtegminal." 



Orals. 



The orals and adjacent side plates forming the sides of the angles of 

 the mouth in Calamocrinus are interesting from their structure, differing, 



