55° 



HERRICK E. WILSON 



Thaumatocrinus a striking similarity is discovered. In Thau- 

 matocrinus the interradial arms are innervated by secondary branches 

 of the axial cord, which originate slightly above the point of bifurca- 

 tion of the basal cord, quickly join, and pass up the interradial arms, 

 while the main branches pass up to the radials (Fig. 7, No 2). 

 In Xenocrinus and Compsocrinus (Fig. 8, No. 1) the anal nerve 

 ridge arises at the point of bifurcation of the axial trunk ridges in 

 the posterior basal, and there can be little doubt that the branching 

 of the underlying nerves took place in the same manner as they do 

 in Thaumatocrinus. The parallel here is so close that the writer 

 was at first inclined to the belief that the interpolation of the anal 



series in the Camerata is of the 

 same type as the interpolation of 

 the interradial arms in Thaumato- 

 crinus. In Thaumatocrinus 1 the 

 "interradial" radials appear very 

 early in the ontogenetic develop- 

 ment as narrow plates separating 

 the radials and gradually increase 

 to the size of the true radials. An 

 objection to this form of develop- 

 ment has been stated by Bather, on 

 the ground that no primitive genera 

 have been found in which the anal 

 plate appears as a narrow linear 

 plate. 2 This objection, however, is 

 not formidable, for the change may have been a discontinuous 

 mutation, or may have taken place during periods of retreat of 

 the sea. Ulrich believes that most mutations have so taken place, 

 for he says in the "Revision of the Paleozoic Systems": " . . . . 

 almost invariably we deal with the nearly finished product of a 

 process of mutation that was begun and established before the 

 new phase invaded areas now accessible to the student of fossil 

 faunas." 3 



1 See Ref. 16, p. 337. 



■ Ref. 3, fifth notice, p. 37. 



J Ref. 36, pp. 498-501. 



Fig. 8. — Diagram showing the 

 course of the radial and anal ridges 

 in Compsocrinus and the branching 

 of the nerves in Thaumatocrinus: 1, 

 Compsocrinus harrisi (based upon 

 Wachsmuth and Springer); 2, 

 Thaumatocrinus renovatus (after Car- 

 penter) ; course of nerves based upon 

 dissection by the writer. 



