1885.] NATURAL SCIENCES OF PHILADELPHIA. 231 



their external form agree with the higher radials, being only 

 narrower, and free plates. 



In the Inadunata, in which the arms are free from the first 

 radial, the homologies of the outer primary radials, which we 

 have distinguished as brachials, vary in some genera considerably 

 in number. The greatest variation in this regard occurs among 

 the typical Cyathocrinid^e (Cvathocrinites as we called them here- 

 tofore), in which the number varies even among the rays of the 

 same species, so much indeed, that one ray may have one, the 

 adjoining two, the next perhaps five or six. The other groups 

 have rarely more than two brachials, and most of them but one. 

 The brachials are regular arm plates, with a well-developed ambu- 

 lacral furrow, but without pinnules. The arms are composed of 

 single joints, except in the Poteriocrinidae and Eucrinidae, in 

 which the biserial arm structure is associated with the uniserial 

 one. Pinnules are wanting in the Hybocrinidse, Symbathocriniclse 

 and Cyathocrinidae, and in the two former the rays are undivided, 

 consisting of a single arm. In the Heterocrinidae and Belemno- 

 crinidae, the pinnules are arranged from every second or third 

 joint throughout the entire arm, the non-arm-bearing joints being 

 united by syzygy, while among the Poteriocrinidae every joint 

 from the second up, bears a pinnule. 



C. The Interradial, Inter axillary and Interbrachial Plates. 



The interradial plates occupy the intermediate spaces between 

 the primarj' rays; the interaxillaries between the main divisions 

 of the ray ; the interbrachials between the arm bases. All these 

 plates may be considered as parts of the same element. The 

 interradials consist primarily of five single plates, which rest 

 either upon the upper or between the lateral margins of two first 

 radials. Only in the genus Briarocrinus, and in a few Ichthyo- 

 crinidsB,do the interradials commence higher up. Higher orders 

 of interradials are only found in the Camarata and Articulata. 

 Their office is to increase the capacity of the visceral cavity by 

 incorporating the lower arm-plates into the calyx, and also to 

 strengthen it. The}- are auxiliary pieces, and serve to fill up 

 spaces, and in this capacity adapt their form to adjoining plates. 

 The higher interradials do not possess the morphological import- 

 ance of the primary ones, which are early developed in the young 

 individual, and represent important elements throughout the 



