290 Graban — Biserial Arm in Certain Crinoids. 



The arguments which this paper attempts to support maybe 

 hrieilj stated as follows : 1. So far as observations have been 

 made on perfect material, new arm plates introduced at the 

 tip of the growing arm are uhiserial. 2. The apical plates, at 

 least in the less specialized biserial species, are rectangular, 

 and cliange with further growth to wedge-shaped and later to 

 biseriality. 3. This is not primarily an old age character, since 

 this condition is found in the apical arm plates of young cri- 

 noids. (See tig. 4.) 



The follow^ing theses are suggested, but not proved. They 

 may be borne in mind in further study, a. In specialized or 

 accelerated genera the newly added plate onay be wedge- 

 shaped instead of rectangular, h. In highly specialized genera 

 the newly added plates may be biserial. c. In old age indi- 

 viduals the last added plates may never pass beyond uniserial- 

 ity, the animal losing the power to further modify the plates. 

 d. In extremely phylogerontic types the biserial condition of 

 the plates may be dropped out, and a uniserial type thus 

 derived from a line which at its acme was supplied with biserial 

 arms. 



The Case of JEJncrinus liliiformis. 



In this species the uniserial condition of the terminal plates 

 was first noted in a perfect specimen belonging to Professor 

 R. T. Jackson of Harvard. Other perfect specimens showing 

 the uniserial terminal plates were studied in the collection of 

 the Boston Society of Natural History and in that of the Uni- 

 versity of Rochester. The first brachial of Encrimis lilii- 

 fortnis is nearly rectangular, while the second is axillary. 

 Above this the 1st plate of the divided arm (No. 1 in tig. 1) 

 is roughly an oblique parallelogram, followed by a somewhat 

 wedging plate, the thinner edges of the wedges being directed 

 towards each other in the two arms of a series. This much 

 may be considered as incident upon the division of the arm. 

 Above this, however, are three plates of the primitive rec- 

 tangular t}^3e, after which (the 6th plate of the separate arm) 

 the plates begin to wedge out, quickly becoming shorter (the 

 7th plate only rarely extends all the way across the arm), 

 and by the 9th or lOth plate normal biserial conditions are 

 established. It is not until the 16th or ITtli plate, how- 

 ever, that the characteristic tubercles appear on the inner 

 side of the plates. At tirst these tubercles are faint, but they 

 quickly become of normal strength. 



In tigure 1, the right member of each of two arm groups is 

 represented. In a^ normal l)iserial conditions continue up to 

 the T9th j^late, after which the plates become more wedging. 

 Not, however, until the 85tli plate is reached are uniserial con- 



