1890.] NATURAL SCIENCES OF PHILADELPHIA, ^ 345 



THE PERISOMIC PLATES OF THE CRINOIDS. 

 BY CHARLES WACHSMUTH AND FRANK SPRINGER. 



Ill nearly all Crinoids, recent and fossil, in which the free arms 

 ■do not start out directly from the (first) radials, the lower arm joints 

 are incorporated into the calyx either by soft tissues, or by means of 

 plates to which the term interradials has been applied. The inter- 

 radials are exceedingly variable in form and character, being in some 

 groups well developed, rigid plates ; while in others they are irreg- 

 ular, ill-formed pieces or mere limestone particles resting within soft 

 tissues. The great difference in their structure among different groups 

 led to the belief that the rigid and regularly arranged plates, which 

 are so characteristic of the Camerata, did not belong to the same sys- 

 tem of plates as the irregular, small pieces which unite the rays of 

 recent Crinoids, and Dr. P. H. Carpenter applied to the former the 

 term " calyx " interradials, as opposed to the interradial plates of 

 the disk. 



A somewhat similar distinction has been made respecting the plates 

 which form the ventral pavement. The heavy, rigid plates of Pal- 

 aeozoic forms were called " vault" plates, the small, irregular pieces 

 of later and recent Crinoids " perisomic " or " disk " plates. The 

 term " vault " was applied generally in cases in which mouth and 

 food grooves are permanently closed, and " disk " where mouth and 

 food grooves are opened out. 



In the Camerata the interradials are arranged on a definite plan. 

 They are stout, large, and united by close suture, so as to make the 

 whole test to the bases of the free arms extremely rigid. In the 

 Actinocrinidae the interradial series invariably commence with one 

 plate which is followed by two in the second row, and two, three or 

 four, according to species, in the succeeding ones. The posterior 

 interradius is wider and split vertically into two halves by a series 

 of anal plates which support the anal tube. In the other families of 

 the Camerata, the Reteocrinidae excepted, the arrangement of the 

 interradials is similar and equally regular. But in the Platycrin- 

 idae and Hexacrinidae the first row consists of three or more pieces, 

 and in the Rhodocrinidae the first interradial is interposed between 

 the radials. The plates forming the ventral side of the calyx are as 

 rigid as those of the dorsal side, and none of them are described as 

 pierced by water pores. They consist normally of five irregular 



