238 PROCEEDINGS OF THE ACADEMY OF [1885. 



earlier Crinoids. The interradials increase by age, vary greatly 

 in number, often in the same species, and even in different rays. 

 There are generally two plates in the second row, but sometimes 

 one or three ; beyond these the arrangement of the interradials is 

 more or less irregular. In the Inadunata the interradials are 

 located exclusively on the ventral side; in the Camarata both 

 dorsallv and ventrally. 



The Reteocrinidse and Acrocrinidse, exceptionally, possess no 

 primary interradials properly speaking. In the former group, all 

 radials, from the basals up, are separated lateralty by numerous 

 minute pieces, without definite arrangement. Acrocrinus has a 

 large belt of small plates, separating radials and interradials from 

 the basals, and the interradial series proper commences with two 

 plates. 



In Part II, p. 15, when describing the structure of the vault of the 

 " Spha?roidocrinidae," we discriminated between true interradials 

 and interradial dome plates, the former as being developed around 

 the dorsal, the latter ai'ound the ventral pole. At that time we 

 were under the impression, and it was the general opinion among 

 naturalists, that the plates of the ventral side in all Crinoids, 

 recent and fossil, constitute a part of the actinal system. It was 

 known to be the case throughout the Neocrinoidea, and among 

 Palseocrinoids we found several genera in which the interradials 

 of the dorsal side are separated from those of the ventral side. 

 In Batocrinus, the higher orders of radials frequently are not 

 separated bj r interradials, as in the case of the primary ones, but 

 join laterally with their fellows, thereb}^ causing an interruption 

 in the interradial series. These cases, however, form exceptions 

 to the rule ; the interradials of the two hemispheres almost always 

 meet each other, and there is no dividing line by which they can 

 be distinguished. 



That the abactinal interradials extend to the ventral side, is 

 well shown by the Platycrinidae and Hexacrinidse, our former 

 subdivisions Platycrinites and Hexacrinites, in which the first 

 interradials occupy the equatorial zone, and all succeeding ones 

 are located ventrally. When we defined these groups, we described 

 the first row of interradials to be composed of a single plate, a 

 statement which is not strictly correct. 



Consulting our figures (PI. t,figs. 5-8, and PI. 9, fig. 6), it will 

 be seen that in the Platycrinidse and Hexacrinida 1 , the first row 



