THE HOMOLOGIES OF THE ARM JOINTS AND ARM DIVI- 

 SIONS IN THE RECENT CRINOIDS OF THE FAMILIES 

 OF THE COMATULIDA AND THE PENTACRINITID.E. 



By Austin Hobart Clark, 

 of the United Slates Bureau of Fisheries. 



Hitherto, most writers on the recent crinoids have considered the 

 arms as beginning with the tirst joints beyond the ("primary") 

 radials; but so far no one has pointed out the exact relations between 

 the arms and arm joints of the different genera and families. 



Dr. P. H. Carpenter, in his most admirable essay on the genus 

 "Actinometra " a (i. e.. Comaster and Comatula), pointed out that in 

 the Comatulida the first two joints beyond each axillary are always 

 articulated in the same way as the two first post-radial joints, no 

 matter how many axillaries may intervene between the radials and 

 the free, undivided arms. He does not here mention the genus Eu- 

 diocrinus, as understood by him, but in his monograph of the recent 

 stalked crinoids h he says: 



In the five-armed Eudiocrinus indivisus the next joints beyond the radials are 

 syzygial, with pinnules on the epizygals, which clearly shows that they imisl 

 be considered as arm joints and not as belonging to the calyx, although they 

 undoubtedly represent the so-called second and third radials of a ten-armed 

 erinoid. The other species of Eudiocrimis have these two primitively separate 

 joints not united by syzygy but articulated, just as in Thaumatocrinus. The 

 second one bears a pinnule both in Thaumatocrinus and in Eudiocrinus varians; 

 but in Eudiocrinus semperi and Eudiocrinus japonicus the first pinnule is on 

 the fourth joint after the radial. This would correspond to the second 

 brachial of a ten-armed erinoid, hut it is really the fourth brachial in Eudio- 

 crinus. Lastly, in Perrier's Eudiocrinus atlanticus c the tirst pinnule is on the 

 fifth brachial, which corresponds to the third brachial of an Antedon. 



° On the genus Actinometra, Mull., with a morphological account of a new- 

 species (A.) polymorpha from the Philippine Islands, Trans. Linn. Soc. (Zool.), 

 [2], II, pp. 1-122, pis. r-vni, (1879). 



6 Report upon the Crinoidea collected during the voyage of H. M. S. Challenger 

 during the years 1S73-187G : PL 1, the Stalked Crinoids, Challenger Reports, 

 vol. XI of Zoology, p. 47 (Lss4). 



c In reality the first pinnule in semperi. japonicus, and atlanticus is on exactly 

 the same joint; but Perrier considered syzygial pairs as two joints, Carpenter 

 as a single joint "with a syzygy; " hence the confusion. 



Proceedings U. S. National Museum, Vol. XXXV— No. 1636. 

 Proc. X. M. vol. xxxv— OS S 113 



