190 PROCEEDINGS OF THE ACADEMY OP [1881. 



sent the basals or genitals, and the radial dome plates, the radials 

 or oculars. The centre piece may perhaps be compared with the 

 underbasals, or the subanal plate of the Echini. 



The apical dome plates, as the apical plates of the cal^-x, are 

 fundamental elements, and are represented in the vault of all 

 Sphffiroidocrinidse both in the 3^oung and the adult, from the 

 Lower Silurian to the Subcarboniferous. They are generallj' 

 larger than the other dome plates, and more prominent, frequently 

 nodose or spiniferous, though in some species they cannot, at 

 least in mature specimens, be readily distinguished from the other 

 dome-plates which have attained equal size. In some genera, 

 for instance, CuUcocrinus, Miiller, they occupy almost the entire 

 ventral disk ; in Glyptocrinus and Bhodocrinus^ on the contrary, 

 they fill only the median part. In some species of Dorycrinus 

 the central piece is spiniferous and the radials nodose ; in others, 

 all these plates are spiniferous. In Amphoracrinus only the four 

 larger proximal dome plates are nodose or spiniferous ; in Agari- 

 cocrinus allapical plates are tuberculose ; in Batocrinus the entire 

 dome is composed of nodose plates. The proximal plates in some 

 species are attached to the centre plate, in others separated from 

 it by a ring of small accessory pieces, and in still others the centre 

 piece is entirely isolated by the wide belt of minute pieces. The 

 latter is frequently the case in large specimens, and in genera with 

 but few primary arms, like 3Iegistocrinus. In this genus it is inter- 

 esting to find in very young specimens and in the smaller species 

 the central and proximal plates in contact, while in the larger and 

 adult specimens all are isolated, even the proximal plates being 

 separated from each other. The radial dome plates are sometimes 

 attached to the other apical plates, frequently so in young speci- 

 mens, and generally in the Platycrinidaj. 



In Platycrinus the radial series of the dome is composed of 

 two rows of pieces alternately arranged, which decrease in size 

 toward the arm bases, and of which the fii-st and larger plate fits 

 in the angle of two adjacent proximal plates. 



The vault of the Platycrinidtie differs in several particulars 

 from that of the other Sphseroidocrinidoe, and in these same char- 

 acters it approaches the Cyathocrinidte. We elsewhere suggest 

 that the Platycrinoid is the simplest form of the Sphiieroido- 

 crinidse, and that it represents the j-ounger stage of the famil}'. 

 This is indicated by the construction of the calyx, but not less b}^ 



