1881.] NATURAL SCIENCES OF PHILADELPHIA. 183 



It is apparent from these facts that neither the existence of 

 underbasals, nor the modifications which took place in the basal 

 disk, had any such corresponding effect upon tlie general structure 

 of the crinoids as to entitle them to be considered characters of 

 family importance, though in distinguishing subordinate groups 

 they may possess some value. The radial and interradial plates 

 are elements of far greater value. 



2. Radial Plates. 



In our nomenclature we have proposed different terms for 

 special parts of the ra}^, discriminating between radials, brachials 

 and arm plates. We designate as " radials " the whole succes- 

 sion of plates above the basals radially situated and enclosed 

 within the body walls. The "arm plates" form the movable 

 portion of the ra^- ; the "brachials," while radials in position, 

 are ai'm plates in construction, being free and distinctly articu- 

 lated. The term " brachials " is therefore purely a conventional 

 one, employed for greater convenience in description. We desig- 

 nate as "primary radials" those below the first bifurcation, 

 while the " secondary radials " compose the first branches of each 

 ra3% the so-called " Distichalia" of Miiller and other writers ; and 

 those of succeeding bifurcations are distinguished by referring 

 them to the order in which they stand in succession. Miiller, in 

 applying the term " distichalia," was evidently not aware, that 

 there are many fossil Crinoids in which these plates give oflT 

 branches which likewise form a part of the test, otherwise he 

 would not have regarded the arms as commencing at the "axil- 

 lary distichals," 4 



Schultze, in his Monograph Echinod. Eifl. Kalk, p. 5, improved 

 upon Miiller's views, and asserted that "the commencement of the 

 arms begins invariably with the first distinct articulation of the 

 ra3^" Like Miiller, he uses the term " distichalia " for the 

 secondary radials, and proposes no name to designate the plates 

 of the higher branches within the body. Schultze difllers from 

 Miiller in designating the free radials as arm plates, and in this 

 he agrees with us ; except that we distinguish those arm plates 

 which are in direct vertical line with the radials as " brachials." 



De Koninck and Lehon regarded the arms as beginning from 

 the first bifurcation in the ray, no matter whether they became 



