DESCRIPTION OF A NEW CRINOID. 97 



responding anal plates, there being only 1x5 primary radials exposed 

 along the walls laterally. Had also this ring of plates been inclnded, 

 and none of the lower plates been exposed to view, the formula would 

 resemble that given by Angelin: 1x10 primary radials, and 15 to 16 

 parabasals, which latter, of course, would be found to represent inter- 

 radial and anal plates. The Swedish species, however, has more arms 

 than the American, and hence higher orders of radials. I feel very 

 certain that Polypeltes has 3x5 primary radials, the usual number of 

 basals, and no underbasals, that the wanting plates were covered up in 

 its concave lower part, and that it is identical with Corymbocriniis, only 

 representing a more concave form. This is further indicated by the 

 fact that both have the very same arm structure. 



The plates, also, of the ventral side, are, from a morphological stand- 

 point, exceedingly interesting. The so-called apical plates are not, as 

 usually, in contact, but are separated by a number of small plates. 

 The belt of pieces surrounding the central one consists of several rows, 

 which increase in number in larger specimens. Similar ])lates separate 

 the proximals from each other, all occupying an isolated position. 



In defining Acrocrinus Wortheni, Geol. Rep. of Illinois, Vol. VII., 

 p. 343, with diagram, I described a belt of pieces which, under similar 

 conditions, surround the basals, and asserted that these abactinal plates 

 were mutually homologous with the actinal pieces surrounding the 

 central plate. A comparison of the diagram of Acrocrinus with the 

 figure of the ventral side in Mc!s:istocri7iiis concavus will confirm this. 



[Proc. D. a. N. S., Vol. IV.] 13 [Dec. 13, 1884.] 



