366 PROCEEDINGS OF THE ACADEMY OF [1890. 



SO upon the disk of the Ichthyocrinoid genus Taxocrinus, as shown 

 by our figure in the Proc. Acad. Nat. Sci. Philadelphia, 1888, 

 PI. 18, fig. le. In this genus, contrary to the preceding forms, 

 mouth and food grooves are opened out, but otherwise its ventral 

 structure agrees so closely with that of the young Platycrinus sym- 

 metricus that it seems as if there could be no doubt that the two in- 

 teguments are composed of the same elements. Indeed, a slight re- 

 ceding outward of the posterior oral, and movable covering pieces, 

 would place the two forms essentially in the same condition. It 

 would seem to suggest further from analogy, that in all other groups 

 in which the ambulacra are exposed ver}' similar conditions prevail, 

 and that in all those forms, as in Taxocrinus, the ventral surface is 

 a disk. So far we have met with no serious difficulties, but they 

 arise when we consider those forms in which the ambulacra enter 

 the surface at a point from beneath the interradial plates. 



If there was in these forms, as heretofore supposed, a system of 

 skeletal plates distinct from the disk, which cover the ambulacra 

 and the disk generally, it would seem to follow that the upper inter- 

 radials, w'hich surround the orals and cover the ambulacra, must be 

 vault plates, and all lower ones, so far as the ambulacra are exposed, 

 disk plates. 



Cases in which the calyx ambulacra pass out from beneath the 

 interradials before they enter the arms, are found in different fami- 

 lies of the Camerata. They occur more frequently among Silurian 

 than among the later Crinoids, and generally in forms in which the 

 ventral surface is paved by small irregular pieces, such as Ghjpto- 

 crinus, Reteocrinns and Archaeocrinus, but also occasionally among 

 Subcarboniferous forms. A most instructive case of this kind is rep- 

 resented by a rather young specimen of Megistocrinus nobiUs, in 

 which not only the covering plates, but also well developed side pieces 

 enter the calyx. The ventral side of this species consists of moder- 

 ately large, irregularly arranged plates, which gradually decrease 

 in size toward the arms. The tegmen is perfectly flat except near its 

 outer margin, where it-is distinctly plicated to form the large openings 

 for the ponderous arms. At the flat inner portions the ambulacra are 

 concealed, but along the plicated outer j^art the covering plates and 

 side pieces are in sight for some distance, and the interradial dome 

 plates extend only to the lateral margins of the ambulacra. It is 

 now quite instructive that in another, more adult specimen of this 

 species, those parts of the ambulacra which in the former specimen 



