416 



MR. F. A. BATHER ON PETALOCEIIfUS. [Aug. 1 898, 



Fig. 5. 



-Petalocrinus. 

 Diagram of arm-facet. 



clearly-outlined depressions, apparently the traces of plates. A 

 wax squeeze (PI. XXYI, fig. 47) indicates that they are covering- 

 plates alternating over the grooves, and resting on their edges in 

 such a way that, if continued, the whole ventral surface of the arm- 

 fan would have heen covered hy a plating of minute alternating 

 ambulacrals. They do not form, quite regular biserial rows, but 

 occasional smaller plates appear intercalated. 



At the proximal end of the arm-fan is a well-marked facet, which 

 in P. mirahilis we know to be for articulation with the preceding 

 IBr^ ; in other species there may have been more IBr, or the arm- 

 fan may have articulated immediately with the radial, but neither 

 of these suppositions is probable. The 

 following description applies to P. vis- 

 hycensis (PI. XXY, figs. 16-22, & 

 text-fig. 5). The plane of the facet 

 forms, with the plane tangential to the 

 main surface of the arm, an angle of 

 55° in f, 60° in j, 61° in g, Qb° in c, 

 and 69° in d. The facet is bounded 

 above by a straight line, merging at 

 either end into angular processes formed 

 by the ends of the outer side-walls of 

 the outermost grooves. This line marks 

 the widest part of the facet. The dorsal 

 contour is a curve, circular or elliptical 

 according to the ratio of depth to width of the facet, which ratio, as 

 shown by the measurements in the table (p. 422), varies from | to |. 

 In the median vertical line of the facet, about ^ the distance from the 

 ventral margin, opens the axial canal {ax. in text-fig. 5). In weU- 

 preserved adult specimens the lower margin of the facet is raised in 

 a slight rim. From the axial canal down towards this rim run two 

 strongly-marked but short ridges, each at an angle of about 45° with 

 the vertical (r in text-fig. 5). The ridges are broad, and their upper 

 surfaces are seen in d to slope slightly inwards to the median line of 

 the facet. There are thus marked out on the facet three depressed 

 areas, of which the median dorsal one (l in text-fig. 5) is the 

 deepest, and by homology with recent crinoids may be regarded 

 as the ligament-fossa. The two lateral areas (m in text-fig. 5) are 

 separated one from another by a very slight elevation ventral to 

 the axial canal, and may be called muscle-f ossse. The ridges are less 

 marked in the young, but the fossae can always be distinguished. 



This form of facet may be compared with the trifascial articula- 

 tion between certain brachials of Batliycrinus} It seems to follow 

 from it that the arm-fan was not capable of much movement in the 

 vertical plane normal for other crinoids (l\! in text-fig. 5) ; indeed, 

 such movement was hardly needed, since the rigid fans could not 

 have folded up over the tegmen, and would also have got in each 

 other's way if vertically depressed beneath a certain level. On the 

 other hand, this articulation lent itself to movement towards right or 



^ See P. H. Carpenter, Challenger Report on Stalked Crinoids (1884), 

 pp. 8 & 9. 



