12 PROCEEDINGS OF THE ACADEMY OP [1886. 



bases until they meet the tegminal plates on the ventral side. 

 The arms divide into numerous branches, which are connected 

 throughout their entire length, in such a way that the brachial 

 apparatus of the whole animal is included in five broad, net-like 

 leaves with inroUed edges, which when closed overlap each other. 

 These leaf-like arms are separated by very prominent, deep peta- 

 loid areas, whose margins are formed by the deeply incurved 

 lateral faces of the peripheral arm joints, and the radials which 

 are enormously thickened. When spread out, these connected 

 ra3'S would be free from contact with one another, and would 

 resemble the outstretched leaves of a pentapetaloid flower, but 

 folded together they overlap just as the closed leaves of a bud or 

 flower. The arm joints are disposed in regular dichotomizing 

 longitudinal rows, and also in regularly concentric transverse 

 rows, the alternate arrangement of joints of adjacent branches 

 being absent here. The arm branches are transversely com- 

 pressed and flattened, very deep in proportion to their width, 

 meeting throughout their entire length by their lateral surfaces. 

 Upon the dorsal side, the branches are laterally connected by 

 points of attachment projecting from the middle of each point, or 

 by direct contact of the sides of the joint, in such a way as to 

 produce a reticulated structure composed of numerous small 

 meshes. The projection of the points of attachment from the 

 middle of the joints gives them the appearance, when seen from 

 the dorsal side, of a cross with short arms, and the network ap- 

 pears to have regular cross-shaped meshes. At the lower part of 

 the arms the joints are not cross-shaped, but meet directly by 

 their sides, and the reticulate appearance is not so marked as 

 higher up. The whole structure of these leaf-like ra^^s is adapted 

 to the greatest flexibility in all directions, and we think it prob- 

 able that the meshes of the network were occupied by elastic 

 connective substance of some kind. The joints are flat on the 

 dorsal side ; near the calyx they are extremely thick from the 

 dorsal to the ventral side, the thickest being those which rest 

 upon the radials, and they decrease rapidly toward the extremities 

 of the rays. The articular faces at the ends of the joints are 

 provided with an axial canal. Along the ventral side the joints 

 are deeply excavated for the ambulacral furrow, which ramifies 

 with the arm branches throughout their length. The furrow is 

 lined at each side by a row of small side pieces, three to foiir to 



