DISCOVERY OF THE DISK OF ONYCHOCRINUS 471 



fourth is pushed partly under the others, so that not much of it can 

 be seen in this view. They are triangular, and fit closely together 

 at the sides, forming a small pyramid. As they lie in the bottom of 

 the calyx they seem to stand somewhat erect, but in their natural 

 position, with the disk normally distended, it is probable that they 

 would appear more flattened, like the orals in the recent genus Holo- 

 pus (Plate V, Fig. 8). They were perhaps also movable, capable 

 of opening and closing at the will of the animal. These small orals 

 are not always so regular as this. Fig. 4, Plate IV, which is an enlarged 

 view of the central part of the disk in another specimen, shows the 

 irregular, elongate, toothlike appearance which they sometimes pre- 

 sent. The rounded object at the right of the three linear plates is 

 evidently a foreign body, but the other four suggest a stage of almost 

 complete resorption. 



The ambulacra are composed of relatively heavy, elongate plates. 

 They meet by close sutures, and are convex on the outer surface. 

 The inner surface is marked by keel-like elevations, which may have 

 served as lines for muscular attachment. They extend from the oral 

 center along the rays to the point of bifurcation, beyond which they 

 have not been traced; but they no doubt follow the arms in a modi- 

 fied form. The three anterior ambulacra pass in between the bases 

 of the four small orals in a similar manner as they do between the 

 plates of the oral pyramid in Holopus. The two posterior ambu- 

 lacra first meet the outer corners of the large oral — at which points 

 there are some larger plates, apparently serving as braces, or sup- 

 ports, for the tegmen — thence they run along the edges of the pos- 

 terior oral, which sometimes appears to be grooved, and pass in toward 

 the mouth at the junction between it and the smaller orals. Whether 

 these strong alternating plates were covering pieces arching over the 

 food-grooves, or were merely some kind of subambulacral plates form- 

 ing a support for more delicate plates which were not preserved, or 

 for an ambulacral structure composed only of soft parts, I am unable 

 to determine. The variation in size and shape of ambulacrals in 

 the living Crinoids is very great, and there is no reason why plates 

 as large as these may not have served as covering pieces, opening 

 out when the orals did to admit the inward passage of the food- 

 bearing currents. 



The connecting integument between the ambulacra was mem- 



