I \I. WIim'KIM g DIOM] I". 



aa they do, from the oral platea of all other known Neocrinoids.* The oral 

 plates of modern Stalked Crinoida are usually flat, thin triangular plates, 

 occupying the interradial angle between two adjoining food grooves. These 

 oral plates are generally perforated with a few water pores, similar to the 

 pores we find on the rows of perforated perisomatic platea which separate 

 the side plates of the food groove from the stouter imperforated interradial 

 plates above the radials. 



The shape of the oral plate seen from above is convex, with an anterior 

 keel. The general appearance of the oral platea and adjoining side plates 

 (in one side of the angle of the mouth is shown in Figure 7 of Plate IX. 

 The lower part of the side plates is not cleared of the connective tissue 

 uniting them. Seen facing the angle of the mouth, the oral plate is high, 

 irregularly rectangular, with rounded corners, resembling somewhat a short 

 femur ( Plate IX. Fig. LO, o). A prominent rounded keel of dumb-bell shape. 

 -» . extends for about half its length on the anterior face. Seen in profile 

 from the left i Plate IX. Fig. 11). the position of the keel, </, is shown aa it 

 passes rather abruptly into the upper face of the oral and very gradually 

 into the lower part of the oral. A second keel, arrow-head in shape, 

 projects laterally from the face of the oral. The outline of the oral plate 

 opposite the central keel is concave. Seen from the other face, the distal 

 extremity of the oral is more pointed (Plate IX. Fig. 9), and the lateral 

 keel of that face runs in an undulating curve to the lower end of the oral. 

 The orals, as well as the adjoining side plate, are closely and compactly 

 reticulated (Plate IX. Figs. 8-14). The oral plate, o, and its nearest side 

 plates, a, b, c, as they appear when cleaned, are shown in Figure 8 of Plate 

 IX. Figures 9 to 1-4 of the same plate are the oral and the side plates, 

 corresponding to o, a, l>, c, of another mouth angle. The first side plate 

 sometimes carries a keel like the lateral keel of the oral ( Fig. 12). These 

 side plates also seem to vary considerably in outline. Compare Figures 12 



* The persistent oral plates of Holopus. Hyocrinns, Rliizocrinus, and Thaumatocrinus present in 

 each a different stage of development (Chall. Rep , p. 72). The orals are small in Rhizocrinus, and 

 in Calamocrinus comparatively still smaller. They are separated more widely from the edge of the 

 disk by a large and thick perisome covered by a wide belt of interradial plates. They differ from 

 the orals of Hyocrinns and Rhizocrinus in being imperforate: at any rate, it is impossible to detect 

 the water pores in the midst of the limestone reticulation of the so called orals. Carpenter, in a letter, 

 suggests that they may be the fused proximal plates of two ambulacra, as they do not seem to be 

 sufficiently differentiated from the big plates of the ambulacra! skeleton. In all the Pentacrinida? and 

 Comatuh-e as shown by Carpenter, except Thaumatocrinus, the orals eventually undergo a process of 

 resorption ; this " commences in Comatula before the young detaches itself from the larval stem, and 

 no traces of the orals are to be found in the adult.'' 



