DISCOVERY OF THE DISK OF ONYCHOCRINUS 489 



Burlington and Keokuk; until the culmination of the group in Ony- 

 chocrinus, some species of which regularly add another primibrach. 

 It prevailed exclusively during the Warsaw, St. Louis, and Kaskaskia, 

 after structure No. i had disappeared. 



Occasional abnormal specimens occur in these genera, but they 

 are rare, and no more than might be expected to occur in a structure 

 so powerfully modified as this. In order to test the possible impor- 

 tance of these exceptions, I tabulated the facts as observed in two 

 abundant species, representing the two leading genera of plan No. 2, 

 with three primibrachs, viz., Forhesiocrinus multibrachiatus L. & C. 

 (not the Crawfordsville species erroneously so labeled in many col- 

 lections, but from a different horizon and locality), and Taxocrinus 

 spendens M. & G. (which is the well-known Crawfordsville species 

 labeled in some collections T. multibrachiatus, and in some T. Meeki). 

 Both were prolific species, flourishing abundantly in two different 

 horizons and localities — thus furnishing the right conditions to show 

 irregularity if it exists. Of these I have a large number of specimens 

 in which iive rays are exposed. In thirty such specimens of F. multi- 

 brachiatus, ranging from very young to adult, all have the regular 

 three primibrachs throughout, except three specimens, which have 

 two primibrachs in one ray. In T. splendens, out of eighty-nine 

 specimens — 



77 have the regular 3 IBr throughout, 

 4 have 4 IBr in one ray, 



6 have 2 IBr in one ray, 



2 have 2 IBr in two rays. 



All Other rays of these are regular. 



Variations as great as this are to be found in the strongest char- 

 acters of almost any group in nature. Out of a large number of 

 Pentremites godoni I have no less than thirty-two specimens with 

 only four rays, and seven with six rays. I have a Taxocrinus, other- 

 wise perfect, which has only four rays — there being no sign whatever 

 of a fifth. The seeker after good luck will find by diligent search 

 more four-leaved clovers than he will exceptions to the regular brachial 

 arrangement of Taxocrinus and Forbesiocrinus. 



The case of Forbesiocrinus agassizi has been cited as a decisive 

 example of irregularity in this group, being said to vary from one to 



