74 SMITHSONIAN INSTITUTION 



the perisome, and thicken by a repetition inwards of the same diffuse areolar 

 tissue. The Radial system he considers to include the joints of the stem, the 

 centrodorsal plate, the radial plates, and the plates of the arms and pinnules, 

 or brachials. To the Perisomatic system he refers the plates sometimes seen 

 between the second radials, and any other plates of the cup or disk. 

 Dr. Carpenter, 1 while not agreeing altogether with Sir Wyville as to the 

 grounds of differentiation of these plates, substantially recognizes the two 

 systems of Radial and Perisomatic plates as defined by him, except that he 

 ranks the basal plates with the former instead of the latter. 



As before stated, Wachsmuth and Springer divided the plates of the 

 crinoid skeleton into primary and supplementary plates; the former including 

 the stem ossicles, infrabasals, basals, radials, brachials, orals, and ambulacrals, 

 and the latter the anal, interbrachial, and interambulacral plates. According 

 to either of these groupings of the plates the anals and interbrachials fall 

 under the same category. It was also demonstrated by Wachsmuth and 

 Springer 2 that all plates interposed between the rays, from the basals to the 

 orals, whether interbrachial or interambulacral, belong morphologically to the 

 same element. Hence it follows that if in the growing crinoid certain spicules 

 of the ventral perisome developed into well-defined plates, which remained 

 permanently in a definite position in the axils between the radials or brachials, 

 they would become the interbrachial (or interradial) plates as we know them; 

 so that whether a certain form has interbrachials or not depends upon the 

 extent to which the perisome has developed between the rays and their divi- 

 sions. The influence of the varying development of these plates upon the 

 classification of the group has been sufficiently discussed in the chapter on 

 Morphology. 



There is another modification not suggested by anything apparent in the 

 primitive type, but affecting the general form and habitus of these crinoids in 

 a way that is of considerable practical importance. Anyone who has had occa- 

 sion to arrange the fossils of this group cannot help being struck by the pres- 

 ence of two general types. One is marked by the tendency of the calyx and 

 arms to form a globose, ovoid, or pyriform crown, in which the arms lie in 

 close contact — although in some genera the lower part of the rays are 

 separated by wide interbrachial areas, above which they come together again. 

 In the other, on the contrary, the tendency is toward a spreading crown, caused 

 by the increasing divergence of the rays upward. In the first the plates of the 

 rays and arms, and the intervening interbrachial structures when present, are 

 for a considerable distance up more or less flush with one another exteriorly, 

 so that the general curvature of the crown is but little interrupted. In the 



"■Ibid., 1886, p. 742. 



! The Perisomic Plates of the Crinoids, Proc. Acad. Nat. Sci., Philadelphia, vol. 42, 1890, pp. 345-375. 



