STRUCTURE OF THE CRINOIDEiE. 17 



In S. geomefricus the space on the ventral surface surrounded by 

 the arms is proportionally mucli smaller than it is in most other 

 species. The arms stand in a circle which is one half the diameter 

 of the cup. In the centre of this circular space there appears to 

 have been no large plates. The arms are not placed on the edges 

 of the primary radials, but near the centres of the plates, vi^hich 

 have at the point of attachment of the arm in each a small perfo- 

 ration, supposed to be a passage for the brachial muscles. From 

 these points shallow grooves proceed to the open space in the 

 centre. The mouth is situated on one side, between two of thi 

 arms, as represented in figure 2, m. 



In Rhodocrinus there are no grooves upon the ventral surface, and 

 the mouth is at one side, between two of the arms (see figure 3, m). 

 The ambulacral vessels must therefore have passed directly into 

 the interior through orifices at the bases of the arms. 



In Crotalocrinus the mouth appears to be central, and provided- 

 with an apparatus of small plates by which it could be opened or 

 closed. The ambulacral grooves radiate from it, and divide twice 

 or oftener before reaching the bases of the arms. There must be 

 therefore at least twenty arms, one for each branch of the grooves. 

 The grooves are provided with marginal plates, which are repre- 

 sented in the figure closed down, those of one side of the groove 

 interlocking with those on the other side, and thus forming a roof- 

 like covering over the ambulacral vessels. 



SECTION II. 

 CEINOIDE^ OF THE CHAZT LIMESTONE. 



The lower part of the Chazy group consists of sandstones, shales, 

 and impure calcareous rocks, and the upper part of limestone, in 

 general thick-bedded and densely crowded with organic remains. 

 Some of the strata of this limestone are composed almost entirely 

 of Rhijnconella 'plena, others of several undescribed species of Orthis, 

 while many of them are little else than compact masses of the 

 comminuted remains of Cystideans and Crinoids. • 



