630 



GEOLOGICAL SURVEY OF IOWA. 



a. part of the structure of the body, and may be more or less developed, as we have 

 had occasion to observe in different species. 



FIG. 101. 



Illustration of 

 basal plates. 



I would therefore propose a modification of the generic 

 description, so as to include the basal plates, properly s<> 

 called, which, from our present knowledge, appear to be 

 three*. The generic formula, as thus modified, would be as 

 follows : 



Basal plates, 3; 



Subradial plates, 5; four of equal size, one 



on the anal side larger and hexagonal. 

 Radial plates, 3 or 4 X 5; 

 Interradial plates, 4 to 20 or more; 

 Anal plates, 10 to 24 or more; 

 Interaxillary plates, 1 to 10 or more; 



Under this generic formula the following species may be arranged, as well as some 

 others known to me at the present time. 



In the generic description of DE KCWINCK and LE Hox, and in the figures of their 

 species, there is indicated a peculiarity in the mode of articulation of the arm-plates, 

 consisting apparently of an indentation in the upper margin of the plate for the 

 reception of a projection or process from the lower side of the succeeding one. This 

 character, though not always conspicuous, is sometimes a feature of more importance 

 and complexity than is usually observed in the plates of crinoids. The indentation 

 on the upper margin does not extend throughout the thickness of the plate, but only 

 to a very moderate depth, and is filled by a superficial plate which is separately 

 articulated and sometimes anchylosed to the outer margin of the plate above, and 

 lying over the suture below, somewhat like the patella of the knee-joint. This patel- 

 loid plate is sometimes large. 



The following illustration is of a portion of a ray of the Forbesiocrinus agassizi, 

 exhibiting the relation of these patelloid plates. 



* In respect to these lower plates, as well as in the general form of the radial plates, 

 this genus has an analogy with ICIITHYOCRINUS; which, with an external basal series of 

 five plates, has sometimes in the base of the body a range of three plates which appear 

 never to be developed externally. In the FORBESIOCRINUS, the series corresponding to 

 these plates of ICHTHYOCBINUS has become so far developed as to form a ring at the base 

 of the calyx. 



