38 SMITHSONIAN INSTITUTION 



The distribution of the primary nerve cords in the Flexibilia undergoes a 

 significant modification in connection with the inequality of infrabasals, result- 

 ing from the reduction of the primitive five plates to three. The primitive five 

 being equal plates, it would be expected that the two fused pairs would each 

 have an area of two-fifths of the pentagon. But instead of that, measurements 

 of all the detached bases of F orbesiocrinus like those figured on Plate XXX 

 disclose the fact that the two undivided sides of the pentagon formed by the 

 larger (fused) plates are shorter than the other three sides which each include 

 parts of two primitive plates. Fusion of the two pairs has therefore been 

 accompanied by retardation of their growth, or a relative compression. This 

 is found to be the rule not only in the infrabasal pentagons of the other Flexi- 

 bilia, but also in most unequally tripartite bases of monocyclic forms. It is 

 true of the blastoids, of Stephanocrinus, and of the Platycrinidae, as has been 

 proved by actual measurement of over 200 specimens of Pentremites, 50 of 

 Stephanocrinus, and many bases of Platycrinus. Thus while the area of the 

 combined pairs is relatively reduced, that of the single original plate is in- 

 creased, so that this plate is usually found to be considerably more than one- 

 fifth the area of the pentagon; in some cases the increase has been so great that 

 the three infrabasals are not far from equal (Pis. LIII, fig. 2c; LXIII, fig. 4). 



Such a reduction in size of the fused plates would seem to be the direct 

 consequence of anchylosis. 1 Upon lateral fusion of pairs of the original five 

 infrabasals (or basals in a monocyclic crinoid) marginal growth of the com- 

 pound plate ceases along the lost sutures; and as plate growth and growth of 

 the body wall take place only along sutural areas, such growth in the middle of 

 the compound plate is inhibited. Growth in the nerve chambers corresponding 

 to the fused pairs is also partially restricted, so that the funnel enclosing the 

 chambered organ assumes a triangular outline. At the same time the two 

 nerve branches which pass on either side of the lost sutures must be similarly 

 retarded in their growth, and cannot always separate as they do in freely 

 growing areas. Therefore, while the nerves proceeding upward from the 

 chambered organ at those sides of the infrabasal pentagon which are composed 

 of parts of two original plates are in pairs, those from the sides consisting of 

 the fused plates are often apparently single as far as the basal commissure. 

 The single nerve in the compound infrabasals, however, is really a double nerve 

 with the halves closely apposed by means of the resulting compression, and is 

 covered externally by a single ridge; this apposition of the two original nerve 

 cords is subject to variation according to the greater or less degree of the com- 

 pression, and, therefore, it may be found complete or incomplete in forms 

 otherwise closely similar. 



■ Wilson, H. E., Evolution of Basal Plates, Jour, of Geology, vol. 24, 1916, p. 541. 



