MORPHOLOGY 39 



The coalescence of the two primary nerve cords on emerging from the 

 proximal ring seems to be a frequent occurrence wherever pairs of the primi- 

 tive five plates have been reduced to single plates by fusion. A good example 

 among monocyclic forms is found in Stephanocrinus angulatus (text-fig. 4, 

 p. 35) in which the external ridges are extremely well defined; here the ridges 

 pass to the radials singly from the undivided faces of the two fused plates, 

 while upon each of the other three radials a pair of ridges enter, one from each 

 side of the interbasal sutures. Among the entire set of ridges two are always 

 narrower than the rest, and it will be seen from the figures that owing to' the 

 increased size of the odd basal one sutural face of the fused pairs is shorter 

 than the others; it is invariably those shorter faces that are traversed by the 

 two smaller ridges. In this species the shape of the nerve-center is indicated 

 externally by the sharply triangular cross-section of the basal cup. A similar 

 relation of the external ridges appears in the Camerate species Steganocrinus 

 pentagonus (text-fig. 5), where owing to the interposition of an anal plate the 

 base consists of three equal plates forming a hexagon; the ridges passing to 

 the three undivided sides of the hexagon are single, while those entering the 

 other two radials and the anal plate are in pairs astride the interbasal sutures. 

 The consolidation of the original pairs of nerves is subject to variation accord- 

 ing to the amount of compression in the fused plates, and the external appear- 

 ance is often greatly modified by the breaking up or reduplication of the ridges ; 

 but it seems to be a general rule that primarily there is one nerve cord for each 

 sutural face. 



In Lecanocrinus, Pycnosaccus, etc., the chambered organ is strongly tri- 

 angular, and in exceptionally well-preserved specimens we sometimes find the 

 single neural ridge, as in the other cases mentioned, passing externally over the 

 middle of each compound infrabasal. 



Since the memoir on the Crinoidea Camerata was written much progress 

 has been made in the knowledge of infrabasals in the Recent crinoids, which 

 were formerly assumed to be all monocyclic, but in which Wachsmuth and 

 Springer predicated the existence of infrabasals upon paleontological evidence. 

 In addition to the discovery of these plates soon afterward by Bury 1 and by 

 Seeliger 2 in Antedon, infrabasals have now been found in eight species be- 

 longing to the recent genera Isocrinus, Metacrinus, Promachocfinus and 

 Hypalocrinus, by De Loriol, 3 Clark,* Doderlein, 5 Jaekel, 6 and myself. 7 



1 Early Stages in the Development of Antedon Rosacea, Philosophical Transactions, London, 1888, 

 p. 288. 



2 Entwicklungeschichte der Crinoiden, Zool. Jahrb., Bd. 6, Morph., 1892, p. 317. 



3 Rev. Suisse de Zool., vol. 2, 1894, p. 494. 



* Proc. U. S. National Museum, vol. 33, Feb. 29, 1908, p. 671 ; and Monograph of Existing Crinoids, 

 1915, p. 316. 



5 Gestielten Crinoiden der Siboga-Expedition, 1907, p. 20. 



* Sitzungs. Bericht. Gesell. Naturf. Freunde Berlin, 1904, p. 192. 

 7 Proc. U. S. National Museum, vol. 36, 1909, p. 187. 



