24 UINTACRINUS: ITS STKUCTURE AND RELATIONS. 



The discovery of this fact was announced by me in August, 1899. * The 

 two forms of base are shown by the text figures below, viz. : — 



Form M : Monocyclic ; 5 basals and centrale. Fig. 1. 



Form D : Dicyclic ; 5 basals, 5 infrabasals, and centrale. Fig. 2. 



1 2 



The infrabasals of Form D consist of five small, usually pentagonal 

 plates, which rest upon the sides of the centrale, in the line of the inter- 

 basal sutures. Their superior angular points project upward a little dis- 

 tance between the basals, and the upper faces often have a slight petalloid 

 convex curvature, to meet which the lower sides of the basals are corre- 

 spondingly concave. This is the most common shape (PI. II., Fig. 10). 

 Sometimes the infrabasals are very acuminate, and their points project far 

 up between the basals (PI. II., Fig. 11). The infrabasals meet each other 

 by interradial sutures, so that they form a complete ring separating the cen- 

 trale from the basals. The exterior outline of this ring of plates is more or 

 less stellate, and is often very similar to the outline of tlie ring of basals. 

 The centrale in this form is a pentagon with stnaight sides, and its angles 

 are directed interradially. The sutures between the centrale and the infra- 

 basals, and of the latter among themselves, are perfectly distinct in numer- 

 ous good specimens, and tlie plates are frequently found lying slightly 

 apart, with the sutures more or less open, owing to distortion in the process 

 of fossilization (PI. II., Fig. 13, and PI. VII., Fig. 3). 



In Form M the centrale usually has the same shape as in the dicyclic 

 form, but its angles are radial in position, instead of interradial, as in the 

 latter. 



Both of these forms are found in the large colony at Locality No. 1, and 

 the relative frequency of their occurrence there is as follows: Out of 435 

 specimens showing the base — 



244 belong to Form M = 56 per cent 

 191 " " " D = 44 " " 



Crinoids of the two forms of base are often found in close connection, 



* Americau Geologist, Vol. XXIV., p. 92. 



