REVISION OF PALEOZOIC STELLEBOIDEA. 15 



areas, the interbrachial marginals. Or the adambulacrals may enter into the 

 construction of these areas, forming interbrachial adambulacrals. When the adam- 

 bulacrals and inframarginals are separated by small plates, the latter are called 

 accessory interbrachials; they may extend almost to the distal ends of the rays. 



Interradial. 



A point half-way between the perradii. The interambulacral areas are inter- 

 radial in position. See Perradial. 



Interradial plates. 



These abactinal plates are rarely retained in living mature starfishes. In many 

 Paleozoic genera they are large, interradial in position, five in number, and 

 lie inside the basal or proximal supramarginals and between the primary radials. 

 They may be homologous with the basals of crinids. They are also known as 



Madreporite ormadreporic plate. 



A sieve-like or spongy plate, with many irregular openings for the circulation 

 of sea water into the so-called "stone canal, " or better, into the water- vascular 

 system. It is commonly granular or striated, and in Asteroidea is abactinal 

 on the disk, basally situated between two rays, but in the Auluroidea is prob- 

 ably always actinal in position. In recent multiradiate forms there are species 

 with more than one madreporite. 



Marginal plates. 



In general the plates which make up the marginal columns bounding the rays, 

 or disk, or the entire animal. They are parts of the primary skeleton. In the 

 early Paleozoic genera the inframarginal and supramarginal columns are usually 

 not directly superposed, making one column of superposed halves as in Meso- 

 zoic and Recent starfishes, but the former column lies outside or laterally of 

 the latter. The inframarginals are then the true marginals, and form a part 

 of both the abactinal and actinal sides. In the primitive arrangement the 

 inframarginals adjoin the adambulacrals, but in derived forms with large pen- 

 tagonal disks they are separated from the latter by accessory interbrachial plates. 

 The situation of the marginals is, however, not always at the margin of the 

 disk and rays, but they may retain their primitive position beside the adam- 

 bulacrals; this is more especially true in forms having a well-developed pen- 

 tagonal disk. These plates are also referred to as marginalia, supramarginalia, 

 and inframarginalia. 



In some forms the supramarginals are not recognizable as such, while the infra- 

 marginals are well-developed; and in others none of the marginals are distinctly 

 discernible. Sometimes the plates of the inframarginal columns adjoin one 

 another, while those of the supramarginal series are separated by small acces- 

 sory supramarginal pieces. Then the inframarginals may be separated from 

 the supramarginals by ambital ossicles (see Ambital) . A proximal supramarginal 

 is the proximal plate of a column and may not be homologous with the primary 

 supramarginal in primitive forms. Proximal inframarginal is used in a similar 

 way and refers to the proximal axillary plate, two of which of adjoining col- 

 umns occupy an axil or lie on either side of the axillary marginal. The latter 

 in primitive forms alone occupies the angle between the rays. The marginals 

 in large disks without angles may be termed disk infra- and supramarginals. 

 The large disk of some species is formed by the oral crowding of pairs of 

 proximal inframarginals, the interbrachial marginals. 



Measurements. 



The size of starfishes is usually given in millimeters along the greater and lesser 

 radii, that is, from the center of the mouth or disk to the tips of the rays. The 

 symbol for this line is R. The smaller radius is from the center of the animal 

 to the center of the interradii; the symbol is r. 

 50601 Bull. 8815 2 



