PETALOCRINUS MIRABILIS 169 
between the arm bases and the central portion of the outer edge 
of the first radials. No interradials observed on the dorsal aspect 
of the calyx. Ventral aspect unknown. 
Arms triangular, leaf-like, rigid, gently curving dorsally. The 
plates composing them closely anchylosed, no sutures vis- 
ible.* Dorsal side smooth, ventral side covered with longi- 
tudinal, rounded ambulacral grooves, separated by sharp ridges. 
Fig. 1.—Outline view of the arrangement of the plates of the calyx and the rela- 
tion of the arms to the calyx in Petalocrinus mirabilis. 
Two ambulacral grooves start from the base of the arm, these 
bifurcate almost immediately, each of these four bifurcate and 
their branches bifurcate again, making sixteen ambulacral grooves 
at the distal edge of each arm. 
Stem small, about one-sixth of the diameter of the calyx. 
The structure of this crinoid does not conform to that of any 
described family, and it seems necessary to establish a new one 
for its reception. Were it not for the number of basals, the 
calyx alone would unhesitatingly be considered as belonging to 
Platycrinus. The most peculiar feature of the crinoid is its arms. In 
their broad leaf-like expanse they resemble somewhat closely those 
of the Crotalocrinide, but unlike the arms of the members of that 
* The specimens are silicified, and in the process of silicification the sutures may 
have been obliterated. In another condition of preservation they might be observed. 
