J. Rofe — Notes on the Crinoidea. 243 



The column of the recent Pentacrinus (on which these experiments 

 were made) has a pentagonal floriform section, and, as above stated, 

 has the longitudinal fibres distributed amongst the five lobes and 

 the ring round the central canal, but the columns of many of the 

 Mesozoic and most of the PalEeozoic Crinoids are circular in section, 

 and in these the fibres appear to be confined to the part of the 

 column adjacent to the central canal. With respect to pentagonal 

 columns, Messrs. Austin state that " there is one character v^hich 

 distinguishes those from the Lias and more modern strata from the 

 pentagonal columns vp-hich existed in the Carboniferous seas, namely, 

 the former invariably articulate by pentagonal, crenated, star-like 

 forms, while the articulating surfaces of the latter are furrowed by 

 linear strige radiating from a central axis." There is much difficulty, 

 however, in detecting the fibres in fossils, owing to mineralization, 

 which, in a majority of cases, has obliterated organic structure, and 

 it is not very easy to distinguish them, even in the recent column, 

 until the calcareous and the animal constituents are separated. 



A few joints from one of the side-arms of the column were next 

 boiled in liquor potassa (Plate VI., Figs. 4 and 5), with the same 

 action on the membrane, but in these the calcareous plates were 

 slightly elliptical in outline, with a mesial ridge on the transverse 

 axis, and strongly resemble the elliptical plates of the column 

 of the Mountain Limestone Platycrinus. Some connected joints of 

 the same side-arm were decalcified so perfectly that they floated 

 in the calcareous solution, but preserving the membrane on all sides, 

 whilst at each face of the section, when mounted for the microscope, 

 were exhibited, with a low power, two tufts of fibres, one on each 

 side of the median ridge. (See Plate VI., Fig. 7.) 



I may here remark that, in breaking off a portion of a side- 

 arm, these fibres draw slightly from the lobes before they rend, 

 and may, with the help of a pocket lens of moderate power, be 

 distinguished without any preparation. The rays and pinnulse acted 

 on in the same manner give similar results, and indicate a calca- 

 reous skeleton surrounded by a membrane or epidermis, which, 

 in the extreme joints, is almost semi-corneous. The calcareous 

 portions show the groove on the upper side of the ray, and the 

 pinnulse have the same grooved section to their extremity. This 

 may be found also in the pinnulge of many of the Crinoids of the 

 Silurian and Mountain Limestone formations, and probably in the 

 Crinoids generally, but not universally, as Bhodacrintis, for one, 

 appears to be without gooves to the arms. 



These experiments on the recent Crinoids, combined with the 

 structure shown in the screw-stones, appear to prove the envelop- 

 ment of the joints of the column, pinnulse, rays and side-arms by 

 an elastic membrane, capable of allowing a certain limited power 

 of motion. 



With regard to the heads, we have no recent calices to ex- 

 periment upon, but we have heads, and plates from them, so 

 identical in their condition with the screw-stone casts that very 

 little doubt can arise that the plates of that part of the animal 



