110 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
Crinoids, where they exist, are auxiliary pieces, which increase by multipli- 
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 Reteocrinidx 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, 5 ce). 
The interradial plates, as already stated, are continued into the tegmen. 
This may be readily perceived in species which have but one or two bifurea- 
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 Strofocrinus, 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 Strofocrinus, we may refer to that of the 
allied genus Sfeganocrinus, in which in like manner the arms branch off alter- 
nately like pinnules from the two main divisions of the rays; but while in 
Strotocrinus the lower parts of the arms are incorporated imto the calyx, and 
form a continuous rim from which the free arms start off, in Sfeganocrinus 
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 Steganocrinus 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 (S/egano- 
erinus pentagonus, Plate LXI. Fig. 5%). This case is the more instructive, 
because S/eganocrinus, with its free arms, may be regarded as representing 
an early stage in the developmental history of S¢rotocrinus. 
