120 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



so-called radial dome plates, as before suggested, are extravagantly developed 

 covering pieces, but indicates that the "vault" was formed by a gradual 

 extension of the interambulacral areas along the line of the ambulacra, either 

 covering them entirely, or encroaching upon them and leaving the more 

 prominent plates exposed. 



. In Qlyjptocrinus and Beteocrinus the tegmen is essentially in the same 

 condition as in Megistocrinus nohilis, but no orals are distinguishable, and 

 the whole ventral surface, including the median portions, is covered by 

 minute granular pieces. The middle part is evenly convex, but toward 

 the periphery there are ridges leading to the arm bases, and on top of 

 these ridges the covering pieces are exposed. In the median portions the 

 ambulacra were evidently subtegminal, being roofed over by superimposed 

 inter-ambulacral plates, which form a continuous integument over them. 



A careful study of the different tegmens which are found among the 

 various families of the Camerata shows that the ambulacra, as a rule, are 

 subtegminal in species with a high dome or bulging at the arm bases, but in 

 species with a flat or depressed surface they are generally tegminal, or 

 become tegminal before entering the arms. They are exposed in forms like 

 '^ Actinocrinus " quinquangularis^ Hdbrocrinus ornatus^ Marsu23iocrinus depressus, 

 and Glyptocrinus ornatus, — species with a low disk ; but they are necessarily 

 subtegminal in genera such as Cactocrinus, Svphonocrmus, etc., in which the 

 tegmen is high and conical. The condition of the ambulacra, therefore, 

 whether tegminal or subtegminal, does not represent an essential structural 

 feature, but is a natural consequence of differences in the form and construc- 

 tion of the tegmen in the respective species. 



Now if it is true that a plated integument was formed in Glyptocrinus 

 and Megistocrinus by the profuse development of the interambulacral plates, 

 and their gradual fusion along the line of the ambulacra over a part of the 

 disk, it would seem to follow that the more substantial vaults of Acthiocrinus, 

 Batocrinus, and Physetocrinus may have originated in a similar way, and that 

 the body of these forms also was covered by a single set of plates. This 

 seems to be confirmed by a very instructive specimen of Physetocrinus, in 

 which the delicate structure at the inner floor is shown in excellent preser- 

 vation (Plate y. Fig. 14). The specimen has the great advantage of being 

 free from any silicious coating, such as obscured the structure in previ- 

 ous specimens of this kind. The outer surface of the tegmen is composed 

 of moderately large, smooth pieces, of irregular form, closely fitted together 



