252 PROCEEDINGS OF THE ACADEMY OF [1885. 



crinidae bore any relation to the disk of recent Crinoids, but we 

 believe, as strongly as ever, thai their ventral surface was covered 

 In a vault. A vault-paved by small irregular pieces, and folded 



like the disk of recent Crinoids, with elevations following the food 

 grooves, is found not only in Glyptocrinus and Reteocrinus, but 

 also among t he later Actinocrinidse. The surface elevations, which 

 form natural grooves at the inner floor, represent more or less 

 open galleries, which in other forms are produced by a thickening 

 of the plates along the inner floor. Miller's description of the 

 vault of Glyptocrinus would apply equally well to Physetocrinus 

 reticulatus which, as we know from actual observation, has a vault 

 and a well-developed disk underneath. An open disk represents 

 a higher form in the developmental history of the Crinoids, than 

 a closed one. This is shown by Cyathocrinus, in which the vault 

 is gradually replaced by the disk, and it is very improbable that 

 the Eleteocrinidae, which did not survive the Lower Silurian age, 

 attained a higher organization than most of the Carboniferous 

 Actinocrinidse. 



According to Carpenter (Chall. Rep., p. 172), "the vault of 

 Actinocrinus has been developed on the left larval antimer, in 

 exactly the same wa} r as the apical or abactinal system is devel- 

 oped on the right; but the oral system, instead of being limited 

 to five oral plates, as in Neocrinoids, reached a very extensive 

 development, so that in its completest form it represents such a 

 parallel to the apical or abactinal system as is to-be met with in 

 no other Crinoid." A similar view was expressed by us when we 

 wrote Part II of this Revision, but we believe the same thing 

 might be said of other Actinocrinidse and all Platycrinidse and 

 Rhodocrinidse. 



Carpenter, as we have stated, applies the term " vault "to all 

 actinal plates covering the disk and tentacular vestibule, and in 

 most of the Actinocrinidse he regards all interradial plates of the 

 ventral side as t he representatives of the interradials at the dorsal 

 side. However, in a few Actinocrinidse and in the Platycrinidse, 

 Rhodocrinidse and allied groups, he restricts the vault to the 

 central piece, proximals and radial dome plates if such are 

 present, and all other ventral plates he takes to be perisomie. In 

 the Cyathocrinidae and Blastoidea lie limits the vault to the sum- 

 mit plates ; but their interradials, although located ventrally, are 

 said to be abactinal. These interpretations, if correct, would 



