The Quarterly journal of the Geological Society of London . ritish Fossil Crinoids—YIII: Cyathocrinus,^ Ann. Mag. Nat. 6, vol. ix (1892). In P. vishycensis the bilateral symmetry of the arm-fan is, asshown by text-fig. 4, more subject to variation. This is due partlyto the fact that the bifurcations in each half do not always takeplace at the same level, partly to the different number of branchingsthat may occur iu each division. The latter difference may dependto some extent on the different ages of the specimens; but the re-lation is not quite regular, since the levels at which the
The Quarterly journal of the Geological Society of London . ritish Fossil Crinoids—YIII: Cyathocrinus,^ Ann. Mag. Nat. 6, vol. ix (1892). In P. vishycensis the bilateral symmetry of the arm-fan is, asshown by text-fig. 4, more subject to variation. This is due partlyto the fact that the bifurcations in each half do not always takeplace at the same level, partly to the different number of branchingsthat may occur iu each division. The latter difference may dependto some extent on the different ages of the specimens; but the re-lation is not quite regular, since the levels at which the variousbranchings take place vary slightly in different specimens. Text-fig. 4 represents actual observed stages between the youngest known,with 5 branches, and the oldest known with 22. This shows per-fect obedience to the law already stated, which was in fact inferredoriginally from this very diagram, the American specimens servingonly to confirm it. Pig. 4.—Diagram showing branching of grooves in arm-fan ofPetalocrinus vishycensis. semor ^U/t. \/i. [The letters refer to the specimens, and the lines drawn across represent thedistal margin of each. Thus the diagram also shows the stages of the right and left halves correspond, except in P. vishycensis (senior),the line is only drawn half way across.] A total of 28 finials was reached by P. inferior, and possibly byP. vishycensis (senior). P. expansus and P. longus also have 28and 27 finials respectively; but in their case the high number isattained, not by continuous regular dichotomy, but by a remarkablemeristic variation, which may be defined as the addition or inter-calation of another half-fan. This is fully described under thosespecies (pp. 431 & 434). I cannot recall a similar variation amongCyathocrinoidea, but certain crinoids in which the arms are pinnu-late and normally bifurcate once, often have one ramus forkingagain, so that the arm has 3 rami. This varies in individuals andeven in the rays
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