424 Revieus — Wachsmuth Sf Springer's Monograph on Crinoids. 



the unreliability of the law as now stated. Wachsmuth and Springer 

 say that the structui-e in Pentacrinus [= Isocrimis] " simply points 

 to the existence in some groups of transition forms intermediate 

 between Monocyclica and Dicyclica" (p. 66), and again, the 

 structure in G. Foriishelli " proves nothing more than that in this 

 species the monocyclic stage was as yet incompletely developed." 

 At the same time they are not clear whether Monocyclica were 

 derived from Dicyclica, or vice versa, though they favour the former 

 view ; also they find it " difficult to explain the change in the 

 orientation of the stem." 



JNow all these difficulties can be overcome, and the differences 

 between Mouocj'clica and Dicyclica seen in their true light, if we 

 pay rather less attention to the particular shapes of the ossicles, and 

 rather more to the relations of the axial nerve- cords and the 

 chambered organ. The middle drawing (P) in Fig. 1 shows the 

 relations of the cords in a pseudomonocyclic form. What has taken 

 place is a compression, not a torsion : the orientation of the cords 

 both above and below remains j)recisely as in Dicyclica and unlike 

 that in Monocyclica. Confirmation of the hypothesis of compression 

 inny be obtained by cutting transverse sections across the chambei-ed 

 organ of an admittedly pseudomonocyclic form. Figure 3 is 

 a diagram reconstructed from several such sections of Pentacrinus 

 [ = Jsocrin?'s] as figured by P. H. Carpenter in his "Challenger" 

 Eeport. We here trace the nerves as they pass out of the radials 



Fig. 3. — The course ot the axial nerve-cords in Isocrinus. 



into the basals {n 1), then meeting in the middle of the basals and 

 passing down as interradial cords {n 2). Now comes the important 

 point : these cords, instead of passing directly into the capsule of the 

 chambered organ, as iu Monocyclica, fork again : the branches {n 8) 



