496 Observations on the 



The restriction of the number of primary radials to two, 

 in Platycrinus, seems to be a natural one ; but all the 

 genera of the CyathocrinidcB, as now constituted, except 

 one or two lately established upon the characters of the 

 superior parts, are variable in the number of primary 

 radials to each ray. The genus Cyathocfinus, as defined 

 by De Koninck and Le Hon, is restricted to three pri- 

 mary radials to each ray ; yet we find the number in C. 

 rotundatus ? Hall, to vary in the different rays of the same 

 specimen ; their number on the anterior ray reaching seven 

 or eight, and all the others having more than three. In 

 C. viminalis, Hall, there are four primary radials to each 

 ray, and in C. divaricatus, Hall, the anterior ray alone has 

 four, while the others have three primary radials. The 

 other characters of all these species plainly correspond to 

 the typical forms of Cyathocrinns. 



In Poteriocrinus and Scaphiocrinns the number of pri- 

 mary radials to each ray has been regarded as of only 

 specific importance, yet in the latter genus there are 

 usually but two primary radials, with an occasional ad- 

 dition to the number in the anterior ray. All the species 

 * of Zeacrinus yet described, with their superior parts, have 

 but two primary radials to the lateral rays, the anterior 

 one having more; the number being variable in different 

 species. 



If these facts do not show the necessity of a greater gen- 

 eric restriction of the number of primary radials to each 

 ray, they seem to prove that generic distinctions really 

 consist, to some extent, of characters which human lan- 

 guage will fail to convey a definite idea of, but which must 

 be comprehended by careful and numerous comparisons. 

 Those who have studied the crinoidea will probably not 

 regard the latter proposition as altogether unreasonable. 



A general recognition of the generic value of the struc- 

 ture of the superior parts of crinoids, would doubtless 



