160 ORINOIDS. 



the chosen type are diagram matically represented on plate xx, 

 where examples of the leading crinoidal groups are shown. 

 The crown is that portion of the crinoid without the column ; 

 while the part remaining after both stem and arms are removed 

 is known as the calyx. The dorsal cup is the calyx below the 

 point where the free arms are given off- — the disk or tegmen be- 

 ing the calycinal portion ventral to the origin of the free arms. 

 Dorsally there is a zone of basals ( B, B ) ; a ring of radials ( r, r ) ; 

 which often, as in the case under consideration, is separated at 

 one point, posteriorly, by the principal anal plate ( An.). All ra- 

 dially disposed ossicles beyond the primary radials are to be 

 regarded as brachials. For descriptive purposes it is convenient 

 to call those brachials to the first bifurcation costals ( b, b ), or 

 brachials of the first order. The plates between the first and 

 second forkings are distichals {b% b"), or brachials of the second 

 order. If there is further dichotomizing within the calyx, there 

 may be brachials of the third, fourth or fifth orders. The bra- 

 chials not incorporated into the calyx form the free arms ( A, A) ; 

 which give off pinnules (p, p ). Between the several rays are 

 often one or more pieces, the interradials (ir, ir); and between 

 the different parts of the same ray small interbrachials (a, a). 

 Ventrally there can ordinarily be made out five orals, among a 

 greater or less number of smaller plates. 



Our knowledge of the Orinoidea has been vastly expanded 

 through recent investigations among the ancient forms. Many 

 interesting facts have thus been brought to light concerning 

 the skeletal parts of these organisms. And while modern 

 embryology furnishes much information that is important 

 toward a complete understanding of crinoid morphology, a con- 

 sideration of the extinct species is equally suggestive. A num- 

 ber of structures unrepresented in the living feather-stars have 

 been disclosed in this way in all the transitional phases, so that 

 their origin and subsequent role are capable of being traced 

 easily. Certain anatomical features, present only for a short 

 time in the larval forms of the modern types, were in the fossil 

 representatives persistent through life. On the other hand, a 

 number of characters commonly observed in the ancient spe- 



