DISCOVERY OF THE DISK OF ONYCHOCRINUS 507 



d) The modification in the plates of the interbrachial areas might 

 properly have been considered in connection with those of the anal 

 side, inasmuch as they all belong to the system of supplementary 

 plates, as distinguished from that to which the brachials belong. 

 Sir Wyville Thomson was led by his researches on the embryology 

 of Antedon^ to regard the skeleton of the Crinoid as composed of 

 two systems of plates, which he states to be thoroughly distinct in 

 their structure and mode of growth. These he designated as the 

 Radial, and the Perisomatic, systems of plates. The former are dis- 

 tinguished by being chiefly made up of peculiar fasciculated tissue 

 of parallel rods, while the latter commence as simple cribriform films 

 embedded in the outer layer of the perisome, and thicken by a repeti- 

 tion inwards of the same diffuse areolar tissue. The Radial system 

 he considers to include the joints of the stem, the centrodorsal plate, 

 the radial plates, and the plates of the arms and pinnules, or brachials. 

 To the Perisomatic system he refers the basal and oral plates, the 

 anal plate, the interradial plates sometimes seen between the second 

 radials, and any other plates or spicules that may be developed in 

 the perisome of the cup or disk. Dr. Carpenter,^ while not agreeing 

 altogether with Sir Wyville as to the grounds of differentiation of 

 these plates, substantially recognizes the two systems of radial and 

 perisomatic plates as defined by him, except that he ranks the basal 

 plates with the former instead of the latter. 



Wachsmuth and Springer^ divided the plates of the Crinoid 

 skeleton into primary and supplementary plates ; the former including 

 the stem joints, infrabasals, basals, radials, brachials, orals, and 

 ambulacrals, and the latter the anal, interbrachial, and interambu- 

 lacral plates. According to either of these groupings of the plates, 

 the anals and interbrachials fall under the same category. It was 

 also demonstrated by Wachsmuth and Springer^ that all plates inter- 

 posed between the rays, from the basals to the orals, whether inter- 

 brachial or interambulacral, belong morphologically to the same ele- 



' Philosophical Transactions, 1865, p. 540. 



2 Ibid., 1866, p. 742. 



3 North American Crinoidea Camerata, pp. 38-105. 



4 "The Perisomic Plates of the Crinoids," Proceedings, Academy of Science, 

 Philadelphia, 1892, pp. 345-75. 



