102 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



a folding of the test or a thickening of the plates, and which proceed from 

 the middle of the floor to the arm openings (Plate Y. Figs. 13, 14, 15, 16, 

 and 17). Within these grooves are lodged the ambulacra, which are repre- 

 sented by a skeleton of radiating tubes, following the inner surface of the 

 disk (Plate V. Figs. 1 and 10; Plate IV. Fig. 8). The tubes are composed 

 of four rows of plates, two below and two above, both alternately arranged, 

 the former probably representing subambulacral pieces, the latter covering 

 plates. The skeleton has never been found completely preserved, but 

 enough is sliown to indicate that the tubes do not extend out to the centre, 

 but meet in an annular vessel surrounding the mouth. This at least seems 

 to have been the structure of Macrocrinus verneuilianus, in which an almost 

 circular vessel has been observed beneath the centre of the disk at some 

 distance from the inner floor (Plate Y. Fig. 8). No ambulacral tubes are 

 attached to it in the specimen, but it has fiNQ openings in a radial direction, 

 which evidently communicated with the ambulacral vessels. The ring is 

 rather large, and is placed around the contracted upper part of the con- 

 voluted organ, which is also preserved in the specimen. 



The direction of the ambulacral tubes and their branching is well 

 observed in natural casts, in which, after the calcareous parts are removed 

 the ducts of the tubes are represented by filiform elevations upon the 

 surface (Plate lY. Figs. 1, 2, 3, 4, 5), sometimes even showing impressions 

 of ambulacral pieces. It also appears from these casts that the ambulacra 

 in some cases rested directly against the tegmen, and in others lay at a dis- 

 tance from it, only touching the test at the arm bases. The latter was 

 undoubtedly the case in the interesting specimen of Flatycrimis (Plate lY. 

 Fig. 6), in which the ambulacral plates are clearly marked near the margins 

 of the disk, while no traces are found of their inner portions. If the ambu- 

 lacra had touched the inner floor, the delicate markings of the disk plates 

 would not be in sight, but instead of them traces of the tubes would 

 appear upon the surface. This is corroborated by the cast of Borycrinm 

 (Plate lY. Fig. 5), in which the ambulacra are visible to the outer edges of 

 the orals, but disappear underneath them. Also in the beautiful specimen 

 of Cactocrims prohoseidalis (Plate Y. Fig. 10), where the tubes are removed 

 from the floor,* and in Teleiocrinus (Plate lY. Fig. 1), and in Cadocrinus glans 



* Neumayr, who knew this specimen from Meek and Worthen's figure, expressed the opinion that a 

 settling of the tubes probably took place after the death of the animal. He based this opinion upon the 

 structure qf the casts, in which he supposed the ambulacra were always exposed at the surface, which, as we 

 have stated, is by no means the case. 



