52 SMITHSONIAN INSTITUTION 



B. THE SUPPLEMENTARY PLATES 

 a. Interradials and Interbrachials 



It was demonstrated by Wachsmuth and Springer 1 that all plates inter- 

 posed between the rays, from the basals to the orals, whether interradial or 

 interambulacral, belong morphologically to the same element. Hence it follows 

 that if in the growing crinoid certain spicules of the ventral perisome devel- 

 oped into well-defined plates, which remained permanently in a definite position 

 in the axils between the radials or brachials, they would become the inter- 

 brachials (or interradials) as we know them; so that whether a certain form 

 has interbrachials or not depends upon the extent to which the spicules in the 

 perisome developed into permanent differentiated plates in the axils. To this 

 statement, however, exception must now be made as to the permanent inter- 

 radial plates resting upon the basals in forms like Promachocrinus and Thau- 

 matocrinus, which may develop into arms, and are of different origin, probably 

 more in the nature of reduplicated radials. 



The interbrachial structures of the Flexibilia, including those of the pos- 

 terior side called the anals, undergo a greater variety of modification available 

 for taxonomic purposes than any of the other elements of the crinoid skeleton. 

 Speaking first of those belonging to the regular interradial areas, these struc- 

 tures fall into two categories : ( i ) The regular strong and fairly well-defined 

 plates that occur in the axils; and (2) the small, irregular plates or granules 

 studding the pliant integument or perisome which lies between the ambulacra 

 and extends from the tegmen in varying degrees and for variable distances 

 down between the rays and their divisions. When in the descriptions we speak 

 of the " iBr " plates, we mean the former. 



The two kinds of structures are in many ways entirely distinct in their 

 aspect, and the line of demarcation between them is usually perfectly evident. 

 This is ordinarily the case in the Taxocrinidae, but owing to the condition of 

 the specimens in the fossil state the fact cannot always be ascertained in some 

 of the genera. When the rays are closely folded, the perisome is often pushed 

 inward and thus concealed from view, or is so covered with matrix that it has 

 not been exposed; or if preserved, it is frequently destroyed owing to its fragile 

 nature. This has often led to misconception of the real structure, and to mis- 

 representation of the facts in illustrations. For instance,, it was at one time a 

 matter of dispute whether the genus Taxocrinns, as then understood to be 

 represented by T. tuberculatus from Dudley, possessed any connection between 

 the rays beyond a possible single plate. Proper cleaning now discloses, in many 

 specimens of that species, the integument of small plates rising high up between 

 the rays and their divisions (PI. XVI, fig, 5). The interbrachial perisome is 



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



