1885.] NATURAL SCIENCES OP PHILADELPHIA. 269 



later investigations have convinced ns that the so-called orals in 

 all three groups are calyx interradials. At no time, however, 

 have we held these plates to he structurally identical with the 

 proximals. 



Before attempting to determine the identity and relationship 

 of the oral plates in the older Crinoids, it will be necessary to 

 give a full description of the different plates which constitute 

 their summit. 



The central piece, as a rule, is the largest plate of the ventral 

 side. It is not only the centre of figure, but also the centre of 

 radiation, and as such occupies the same position ventrally as the 

 basals occupy on the dorsal side. It is frequently nodose, even 

 spiniferous, but always more or less convex, and has a concavity 

 upon its inner floor, toward which all organs from the arms con- 

 centrate. The central piece is surrounded variously by from 

 seven to twelve other plates ; four of these are larger than the 

 others, interradial in position, and each one rests upon, and con- 

 nects with, one of the four regular interradial series. Toward 

 the posterior side there are three smaller plates (PL 7, figs. 2, 5, 

 and PI. 8, figs. 7, 8), rarely two (PI. 7, figs. 6, 7, 8), which simi- 

 larly connect with the azygous interradius. The three smaller 

 pieces are frequently separated from the larger ones at each side 

 by a good-sized plate, radial in position (Xr in PI. 7, figs. 3-10, 

 and PI. 8, figs. 1, 3); sometimes, however, they unite laterally 

 with the larger ones. This is the case in the simpler forms, such 

 as Symbathocrinus (PL 5, fig. 12), and in Cyathocrinus alutaceus. 

 In very complex genera, and especially among the huge forms of 

 the Actinocrinidpe, the four larger plates are also separated by 

 radial structures, general^ by three plates longitudinally arranged 

 (PL 8, figs. 1, 3, and PL 4, fig. 4), of which the inner ones abut 

 against the central piece, the outer ones against the second radial 

 and against the sloping sides of the four large proximals. In 

 species in which the latter are laterally connected, which is much 

 more frequently the case, there is but one radial plate, and this 

 takes the position and functions of the third one. In species 

 with a single radial, this rests at the three anterior rays within 

 the angles formed by the four large proximals ; while the plates 

 of the two posterior rays are often laterally inserted between the 

 larger and smaller proximals, abutting against the central piece. 

 These two posterior radials were thought by us, and, we suspect, 



