REPORT ON THE CRINOIDEA. 



337 



though Sir Wy\'ille Thomson described a specimen with thirty arms.^ The irregularity 

 in the number of joints between successive axillaries is very striking, especially as 

 compared with the very constant character of the ray-divisions in the Comatulse. 

 Taking for example the genus Actinometra, we find in Pentacrinus decorics the follow- 

 ing series of distichals and palmars which are specially characteristic of different groups 

 of the species of that genus. 



A. Ten arms only ; the first two brachials united bifascially, and 



criiius narcsiamis, 



B. Two distichals united by a syzygy, . 



C. Two distichals, the axillary not a syzygy, 



D. Three distichals, the axillary a syzygy, 



E. Two palmars united by a syzygy, 



F. Two palmars, the axillary not a syzygy, 



G. Two palmars, the axillary a syzygy, . 

 H. Three palmars, the axillary a syzygy. 



the third brachial a syzygy as in Fenta- 

 Group of Actinometra meridionalis. 

 „ Actinometra JiJcesi. 

 „ Actinometra pulchella. 

 „ Actinometra paruieirra. 

 „ Actinometra tijpica. 

 „ Actinometra stelligera. 

 ,, Actinometra multiradiata. 

 ,, Actinometra hennetti. 



In addition to these, Pentacrinus decorus may show numerous combinations of distichal 

 and palmar series such as are characteristic of other groups of Comatulse, and also certain 

 conditions such as two distichals with the axillary a syzygy, which I have not as yet 

 met with in any Comatula at all. These facts well illustrate what has been said above 

 {ante, p. 55) respecting the difference between the arms of Pentacrinidee and Comatulse. 



The rays of Pentacrinus decorum and their subdivisions are sometimes in pretty close 

 contact, though rarely flattened laterally ; while in other cases they are more or less 

 separated by perisome (Pis. XXXV. -XXXVII.). This perisome is sometimes nearly 

 bare and sometimes plated pretty continuously ; and a similar variation is apparent on the 

 upper surface of the disk. This is sometimes covered tolerably closely by rather large 

 plates (PI. XXXIV. fig. 2) ; but the plating, is not cpite so continuous as in Pentacrinus 

 wyville-thomsoni and Pentacrinus alternicirrus (PI. XVII. fig. 6 ; PL XXVI. figs. 1, 2). 

 On the other hand, the gaps between the plates, though sometimes comparatively large, 

 are not so extensive as in Pentacrinus miilleri (PI. XVII. fig. 10). The plates sometimes 

 bear small blunt spinelets which are possibly tactile in function, as they contain branches 

 from the antiambulacral nerves which extend upwards on to the disk fi'om the envelope 

 of the chambered organ in the calyx (PI. LIX. figs. 2-4, ad). The plates bordering the 

 ambulacra of the disk are narrow and spine-like, often forming a kind of palisade, which 

 is more distinct than in any other type of the Pentacrinidaj (PI. XXXIV. fig. 2). 

 They become somewhat irregular on the arm-bases (PI. XXXIII. fig. 6) ; but further 

 out (PI. XXXIII. fig. 4) they begin to show signs of a more or less perfect 

 diflerentiation into the side and covering plates of the pinnule-ambulacra (PI. XXXVII. 

 figs. 23, 24). 



■ Sea Lilies, Tlie Intellectual Observer, August 1864, p. 



(ZOOL. CHALL. EXP. — PART XXXII. 1884.) 



114.3 



