1881.] NATURAL SCIENCES OF PHILADELPHIA. 189 



anal aperture (PL XYIIT, figs. 1 and 9), but their identification is 

 often difficult in forms in which a large subcentral anal tube is 

 interposed between the two small plates, pushing them toward 

 the anterior side, while the central piece rests against the side of 

 the anal tube. (PI. XYIII, fig. 8). 



There are other vault pieces occupying a radial position which are 

 sither in contact with those just described, or, as is more frequently 

 the case, separated from them by a belt of small pieces. Their 

 number varies considerably among species, and depends upon the 

 number of primary arms, without reference to the number of bifur- 

 cations after they become free. They increase in proportion to 

 the number of pi'imarj' arms, in the same manner and on the same 

 principle as the plates of the calj^x, each order of radials has its 

 corresponding plates in the vault. Therefore, in adult specimens, 

 with some practice the number of arms can be ascertained as well 

 from the dome as from the calyx. In species with two arms to 

 the ray, there are two ranges of corresponding radial plates in the 

 dome ; the first or upper being a large bifurcating plate, equiva- 

 lent to the primary radials of the caljx. This is followed by two 

 other plates, which take the place of the secondary radials, one 

 over each arm base, with a third plate — an interbrachial — between 

 them. 



When there are three arms to the ray, there are three ranges 

 of radial dome plates, two plates in direct succession from the 

 large bifurcating plate toward the single arm, and a second bifur- 

 cation, with one plate in each branch, toward the division with two 

 arms. In species with four arms to the ray, there are two secoudarj^ 

 bifurcations, producing radial dome plates of a tMrd order, leading 

 to each arm base, and so on. There are also interradial plates 

 represented in the summit, occupying intermediate spaces between 

 the radials, but their arrangement is very irregular and their 

 number variable. In some genera the number of vault pieces is 

 enormous, notably in Strotocrinus, which has a large number of 

 arms. Looking at such a specimen with its vast number of 

 apparentl}" irregular vault pieces, one would scarcely expect to 

 find this multitude of plates arranged upon a definite plan ; and 

 this the same that prevails in the calj'x. 



We have called the principal plates in the vault apical dome 

 plates, because thej^ correspond to the apical plates of the aboral 

 side. The six proximal plates surrounding the central piece repre- 



