MORPHOLOGICAL PART. 103 



(Plate ly. Fig. 8). That they were attached to the tegmen in many of these 

 Crinoids, and probably in the majority of them, is also suggested by the 

 presence of the so-called radial dome plates. 



The name " radial dome plates " was given by us to a set of large plates 

 in the tegmen, radially disposed, and occupying the regions between the 

 orals and the arm bases. The plates are generally larger than the surround- 

 ing ones, and quite frequently nodose. They were regarded by us, and also 

 by Dr. P. H. Carpenter, until quite recently, as the actinal representatives of 

 the radialSj but later investigations prove they are highly differentiated cov- 

 ering pieces. The plates are either followed by two series of regular cover- 

 ing pieces, and pass out from between the angles of the orals, or they are to 

 a certain extent isolated, surrounded by other plates, and succeeded by simi- 

 lar plates of higher rank. The former is the case in most of the Platycrinidae 

 and Plexacrinidge, the latter among the Actinocrinidae and Rhodocrinidae with 

 subtegminal ambulacra. That the plates in the former case are simply cover- 

 ing pieces, is readily perceived by examining those species in which the 

 plates are but little disturbed. Taking Platycrinus and Dichocrimis, we find 

 in both genera certain species in which two series of small, almost regularly 

 arranged, alternating pieces pass out from the orals to the arms. Such is the 

 case in Flaiycrinus symmetricus (Plate LXIX. Figs. 1 ^, c), in Dichocriniis poly- 

 dadyliis (Plate LXXYII. Fig. 1 h), and in D. lachrymosus (Plate LXXVII. 

 Fig. 2 c). Similar plates under similar conditions also occur in the Silurian 

 Marsupiocrimts (Plate VIII. Fig. 15, and Plate LXXY. Figs. 16 b and 18), and 

 among Silurian genera of other groups. There are no large plates next to 

 the orals, and hence no radial dome plates, unless they are represented by 

 the small proximal plates. From this structure to that of FMycrmus dis- 

 coideus (Plate LXYI. Fig. 10 h), and P. Halli (Plate LXXII. Fig. 7 a\ there 

 is but a short step. The plates in question are comparatively larger, espe- 

 cially the proximal ones, and arranged alternately, although not so regularly 

 as in the former species. In Platycrinus hurlingtonensis ^ however (Plate LXIX. 

 Fig. 3 c), and P. Yandelli (Plate LXYIII. Fig. 3 c), there is generally a large 

 plate followed by smaller ones, and the alternate arrangement of the plates 

 actually commences at the arm bases. If we had only the latter species, we 

 might perhaps be justified in regarding those proximal plates as independent 

 structures ; but comparing them with those of other species in which the 

 arrangement is more regular, it becomes evident that they are all nothing 



