110 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



Crinoids, where they exist, are auxiliary pieces, which increase by multiph- 

 cation in the growing animal, filling up spaces between the rays and their 

 subdivisions. They increase primarily in an upward direction, but partly by 

 intercalation, secondary plates being introduced between the primary ones. 

 It is owing to the intercalation of these secondary pieces that the arrange- 

 ment of the interradial plates in the upper rows is less regular than it might 

 otherwise be. In the simpler forms such pieces are wanting, or only occur 

 close to the arm bases. In some species, however, they are quite numerous 

 in the dorsal cup, as well as in the tegmen, and in the Reteocrinidse they 

 constitute the greater part of the interradial and interaxillary areas. In this 

 family small pieces continually formed in large numbers in the growing 

 Crinoid along the margins of the radials and brachials, and between the 

 primary interradials, so as to isolate these from their fellows and from the 

 plates of the rays (Plate IX. Figs. 1 e, 6 c). 



The interradial plates, as already stated, are continued into the tegmen. 

 This may be readily perceived in species which have but one or two bifurca- 

 tions in the calyx ; but in the more complex forms the primary structure is 

 frequently obscured by the introduction of secondary pieces, giving the im- 

 pression that the plates of the two hemispheres were structures morpholo- 

 gically independent. Looking at a specimen of Strotocrinus, with a broad 

 flanging rim, its hundred and more arms crowded around it, and its thousands 

 of minute "vault" plates, growing smaller outward, and not connected with 

 the interradials of the dorsal side, it is not surprising that such an impression 

 should be created. 



To understand the structure of Strotocrinus, we may refer to that of the 

 allied genus Steganocrinus, in which in like manner the arms branch off alter- 

 nately like pinnules from the two main divisions of the rays ; but while in 

 Strotoeriniis the lower parts of the arms are incorporated into the calyx, and 

 form a continuous rim from which the free arms start off, in Steganocriniis 

 the two divisions of the rays, bearing their small alternate arms, remain per- 

 manently free, and extend out laterally as tubular appendages of the calyx. 

 It is now very significant to find that in Steganocrmiis the interradials of the 

 dorsal cup meet those of the tegmen in such a manner that it is absolutely 

 impossible to draw a line between the plates of the two hemispheres [8tegano^ 

 crinus pentagonus, Plate LXI. Fig. 3 h). This case is the more instructive, 

 because Stegmwcrinus, with its free arms, may be regarded as representing 

 an early stage in the developmental history of Strotocrinns. 



