. Bulletin - United States National Museum. Science. 176 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. fciKk'iuy toward a flattened disk-like form, giving the maximum resistance to wave motion and to oxtornal influences generally; sometimes one, sometimes the other of these tendencies gets the upper hand, depending upon the local conditions of the chosen habitat of the particular type; often the foi-m is modified, as in the so-called irregular urcliins, by the assumption of locomotion in a definite direction, wliich immei^^ I •'^^^Tr : series. Similarlyrcsponsetourgentmechanicalexigencj' has nec
. Bulletin - United States National Museum. Science. 176 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. fciKk'iuy toward a flattened disk-like form, giving the maximum resistance to wave motion and to oxtornal influences generally; sometimes one, sometimes the other of these tendencies gets the upper hand, depending upon the local conditions of the chosen habitat of the particular type; often the foi-m is modified, as in the so-called irregular urcliins, by the assumption of locomotion in a definite direction, wliich immei^^ I •'^^^Tr : series. Similarlyrcsponsetourgentmechanicalexigencj' has necessitated the incorporation of the radials (which correspond to the ambulacrals immediately surrounding the peristomal in the ecliinoids) as a closed ring in the calyx just beyond the basals. Purely mechanical considerations therefore require that the dorsal portion of the most primitive crinoid calyx, which entirely encloses the visceral mass, shall be composed of closed rings of five plates each, these rings being two in number, as two rings offer much greater resistance to outside forces than three or any greater number. These two rings will be the circlet of radials upon wliich the arms are borne, and the circlet of basals, situated between their bases. The infrabasals, which lie on the border line between two (half) meta- meres and are in a way space fillers serving to increase the area of the apical region, will not appear. If by any chance circumstances should arise through wliich the strict operation of these mechanical laws were obstructed or held in abeyance we should expect that at once there would appear in the crinoid calyx addi- tional plates which, far from being new structures or structures appearing for the first time, in reality would be ancient structures long dormant in the crinoid organization awaiting only the relaxation of the strict and closely circumscribed mechanical limitations to reappear in their ancient fashion. It is not at all inconceivable tha
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