22 PLATYCRINIDJE. PLATYCRINITES. 



Column. — The column presents the same structural modifications as in the typical 

 species. 



Miller supposed that the alimentary canal extended down its whole length in the 

 Platycrinites, but this idea has been long abandoned, as the narrow central opening 

 could scarcely have admitted of such an arrangement. 



It is impossible to define the length of the column, either in this or the other species 

 of Platycrinites, as its natural termination has never yet been discovered. It was 

 e\adently of very considerable length, for we have seen them many inches in extent 

 lymg along the exposed surface of denuded limestone strata. 



Auxiliary Side Arms. — These organs are long and delicate, and inserted in the 

 column by articulatuig stria?, in the coniform excavations formed for their reception, 

 ^^^e have seen auxiliary side arms of various species of Platycrinites several inches in 

 length : their office was, we imagine, to aid the animal, by their motion, in sustaining 

 itself in an upright position, as Avell as for the purpose of clasping extraneous objects, 

 by which it was enabled, by retaining its hold, to remain in any desirable position at 

 tlic bottom, and which was suited to its habits. 



Base of Attachment. — This portion of the animal is unknown, but it was probably 

 similar to that of other species of Platycrinites. 



All the plates composing the calcareous skeleton were held together by ligamentous 

 or muscular attachment. Those forming the lower portion of the visceral cup were 

 immoveable, but the plated integument extending over the ca\dty, and lying between the 

 rays and the base of the proboscis, was endowed ■with a slight, but exceedingly limited 

 l>ower of contraction or expansion. 



The meso-plate, from its intermediate size, being smaller than the perisomic plates, 

 and yet larger than those protecting the viscera, would have modified the too sudden 

 change from the rigid portion of the cup, to that which was slightly moveable. The 

 uieso-plate also served to keep the bases of the rays asunder, so that however raj^idly 

 the extremities might have been moved, the shape of the abdominal cup remained 

 unaltered. 



3. Species. Platycrinites mucronatus. (Austin.) 



Definition. — Body globose; dorso-central plate saucer shaped and pentagonal; 

 perisomic plates broad and smooth ; plates arching over the visceral cavity, large, mth 

 central pointed tubercles. Mouth lateral ; rays and column unascertained. 



