374 PROCEEDINGS OF THE ACADEMY OF [1890. 



<listinguislied from the interanibulacral plates, and the mouth, so 

 far as known, is exposed. 



It thus appears that in the Inadunata and Articuhita, as in the 

 Camerata, there is but one integument covering the body, and that 

 the ventral pavement, although undergoing various modifications in 

 geological times, is a true disk. We, therefore, abandon the term 

 " vault " as a morphological term altogether, and consider all plates 

 between the I'ays and interposed between the ambulacra, as well as 

 those covering the ambulacra when these are subtegminal, as plates 

 of the disk. 



We think the plates of a Crinoid fall naturally into two categories, 

 viz. primary, and secondary or supplementary pieces. The primary 

 plates form the fundamental part of a Crinoid, while the supplemen- 

 tary pieces serve to fill up spaces. The primary plates may be sepa- 

 rated into two classes : those developed on the right antimer, which 

 in one way or another are related to the axial nerve cords, and those 

 developed on the left antimer, and connected with the mouth or the 

 annular vessel around it. To the first class we refer the stem joints, 

 basals, underbasals, radials, all brachials whether fixed or free, and 

 the plates of the pinnules ; to the second the orals and all plates of 

 the ambulacra to the end of the pinnules. The remaining plates, 

 which embrace the various perisomic plates, are supplementary pieces, 

 and in our opinion neither strictly actinal nor abactinal. 



Dr. Carpenter, in the July number of the Ann. and Mag. Nat. 

 Hist., p. 12, shows that, in analogy with other Echinoderms, there is 

 fundamentally but one ring of radials in a Crinoid, and that the 

 succeeding plates along the rays are brachials, a view which we have 

 held for some time, and discussed in various parts of our Revision. 

 After correspondence with us and mutual discussion, he brought out 

 special terms for the different orders of brachials. He proposed 

 the term costals for the primary brachials, distichals for the plates of 

 the second order, palmars for those of the third, post palmars for 

 those of the next order. To the arm plates after the last bifurcation 

 he applies the term free brachials. To the most of this terminology 

 we entirely agreed, but in some particulars it does not quite meet 

 the requirement in dealing with the greater complexity and variety 

 of construction found in the Palaeozoic forms. We find it more con- 

 venient to use the term brachials in its general sense, to designate 

 all plates of the rays succeeding the radials. They will be fixed 

 brachials so far as they are incorporated into the calyx, and free 



