400 Prof. Beyrich on the Base [Pelvis) 



axis, in accordance with wliicli the base is symmetrically 

 divided. 



Only in a few ancient Crinoids is the calyx so perfectly re- 

 gular in structure that neither a radial nor a dorsal axis is di- 

 stinguisliable. In others only the radial axis is developed, 

 whilst the base is regularly divided ; and others, again, possess 

 regular radii and a central vertical aperture, with the base 

 symmetrically constituted. Those Crinoids in which only one 

 axis is developed would be divisible into two similar halves 

 from the vertex to the root, through one and the same plane, 

 which in one case would be to l^e drawn through the radial, 

 and in the other through the dorsal axis. But in the numerous 

 Crinoids in which both axes have attained development, it is 

 the rule that the dorsal axis maintains a direction peculiar to 

 it, differing from the radial, but constant in each individual 

 case. The observations to be brought forward in the following 

 pages relate to genera with both monocyclic and dicyclic, tri- 

 partite and quadripartite bases. 



Observation is most simple in the monocyclic tripartite hose 

 of Platycrinus. The symmetrical division of the calyx in 

 accordance with a radial axis is possible in this genus only in 

 species allied to P. pileatus^ Goldf., or P. ruyosus, Miill., in 

 which the calycine aperture has a perfectly lateral position. If 

 such calyces be examined in the reversed position, with the 

 base turned upwards, making the orientation of the pentagon 

 of the base accord %\ath the inten'adius above which the ver- 

 tical aperture is situated, it is easy to see that in all indi- 

 viduals the three segments of the base have the same position, 

 and that the dorsal axis does not coincide with the radial. 



In order to institute more detailed comparisons, it is neces- 

 sary to give the radial axis a fixed position. In what follows 

 I shall, when examining the calyx in the reversed position, 

 turn its interradial pole forward, and indicate it by the letter E, 

 in the explanatory figures, which are always referable to this 

 position. The lateral paired radii and inten-adii I distinguish 

 as the adjacent and abjacent radii and interradii of the right 

 and left side. As by this means the denomination anterior 

 and posterior radii or interradii is avoided, we obtain the ad- 

 vantage that in the upright position, in which observers are 

 accustomed to turn the vertical aperture backward, the expres- 

 sion for the lateral parts remains the same. 



As in tlic pentagon of tlie monocyclic base the angles cor- 

 respond with the inten-adial and the sides with the radial di- 

 rections of the calyx, the law for Platycrinus, illustrated by 

 the annexed figure, runs as follows : — The dorsal axis, d d, in 

 accordance with which the base is symmetrically divided, leads 



