232 PROCEEDINGS OF THE ACADEMY OF [1885. 



discuss this question under Stemmatocrinus, we need not enter 

 upon it here. An anchylosis of the underbasals occurs also 

 in Agassizocrinus by the deposition of new material around the 

 outer surface before reaching maturity, b} T means of which the 

 sutures externally and internally become obliterated. The same 

 is the case with the basals in Edi'iocrinus. 



In cases of three unequal basals, the position of the smaller 

 plate varies among the different orders, but is unchanged in the 

 same one. In all Palaeocrinoidea this plate is located between 

 the anterior and left postero-lateral ray (PI. 6, figs. 21, 25, 26); 

 in the Blastoidea between the anterior and right postero-lateral 

 ray (PL 6, fig. 24); in the recent genus Hyocrinus immediately 

 to the right of the anus (Challenger Report, p. 218). In genera 

 with only two basals, such as Dichocrinus, Talarocrinus, Pteroto- 

 crinus and Acrocrinus, the interbasal suture passes from the 

 posterior to the anterior side (PI. 6, fig. 3, and PL 9, fig. 1). 

 When there are three unequal underbasals, as in the case of the 

 Ichtlryocrinidse (PL 6, fig. 23), and in Tribrachiocrinus (PL 6, 

 fig. 5), the smaller one is placed anteriorly. 



B. The Radial and Arm Plates. 



With the exception of Acrocrinus, the radials proper, the repre- 

 sentatives of the oculars, constitute the first row of plates 

 succeeding the basals, with which they alternate. In most of the 

 Palaeocrinoidea they do not form a continuous ring, being inter- 

 rupted posteriorly by an anal piece, and sometimes by additional 

 plates, while in some groups all five radials are separated by five 

 interradials, so as to form jointly a ring of ten plates around the 

 basals. In the Palaeocrinoidea generally, the radials and their 

 associates are united by suture with each other and with the 

 basals. In Cromyocrinus the union is by syzygy, but in a few 

 of the later Poteriocrinidae those plates are provided laterally, 

 and toward the basals, with more or less deep fossae, which 

 suggest a less close union and a certain degree of mobilit} r . In 

 some species of Forbesiorrin us, Ichthyocrinus and Taxocrinus, and 

 probably in the Ichthyocrinidae generally, the radials were united 

 with one another by muscles; with the interradials, however, by 

 ligament, their lateral faces being provided with deep fossae and 

 dentations along the edges. (PL 5, figs. 3-5). 



In some Silurian genera, the radial at the right posterior side 



