384 PEOCEEDINGS OF THE ACADEMY OF [1888. 



tiary radials, which in C. rugosus, sometimes together with the first 

 plate of the fourth order, rest partly upon the first radial. All of 

 these plates, in various ways, are firmly attached to the first radial, 

 and united suturally with one another, and all of them, by curving 

 upwards and inwards, extend from the dorsal to the ventral surface 

 of the calyx, forming as such a sort of transition between true radi- 

 als and arm plates, in a similar manner as the higher radials of the 

 Platycrinidae, which they resemble in their arrangement. The plates 

 are wedge-form, thinning out toward the dorsal cup, where they are 

 seen as mere points or lines, or one or more of them are invisible al- 

 together. Their lai"ger upper faces, which are exposed ventrally, are 

 deeply grooved for the i*eception of the ambulacra, and, when the 

 covering plates are in position are only partly exposed. The plates 

 above the fourth order are not in contact with the first radials, and 

 may be regarded as true arm-plates, which they resemble in form 

 and in point of mobility. 



The arms are long and branch frequently ; they are connected lat- 

 erally by points of attachment from near the middle of each joint, 

 with open spaces between them, forming together a sort of network 

 around the calyx with innumerable elongate meshes. In C. rvgosus 

 the network is continuous around the calyx, but in C. pulcher the 

 rays are separated, and form five broad reticulate leaves, which, when 

 closed over the calyx, overlap each other, contrary to the case of C. 

 rugosus in which the undivided network is closely plicated and fold- 

 ed. The lower plates of the rays, to the third or fourth order, are 

 immovably connected among each other and with the first radials ; 

 but higher up in the rays, where the plates are no longer in contact 

 with the first radials, an articulation by strong muscles and fossse 

 takes the place of suture. The arm joints, owing to their lateral pro- 

 jections, have the form of a cross with short arms : they are long flat 

 on the dorsal surface, laterally compressed, with straight sides, and 

 deeply grooved on the ventral surface for the reception of the am- 

 bulacra. 



The ambulacral furrows are arched by covering pieces, 3 to 4 

 to each side of the arm joint, alternately arranged. The arm 

 joints are disposed in regular dichotomizing longitudinal rows, 

 as well as in regular concentric transverse rows, the points of union 

 occupying the same line all around. Each arm plate is pierced with 

 a very large dorsal canal, and the bifurcating ones with two, which 

 meet in the middle of the plate ; they ramify to the ends of the arms, 



