5o6 FRANK SPRINGER 



tinued from the Silurian through the Devonian, and into the Carbonif- 

 erous, with constantly diminishing importance, ending, so far as 

 known, in the Keokuk Limestone; but reappearing with the Flexi- 

 bilia Pinnata in the Mesozoic, and continuing to the present time. 

 The three-IBr structure, on the other hand, having barely made a 

 beginning in the Silurian, shows a steady increase through the Devon- 

 ian and Carboniferous, and prevailed exclusively in the Warsaw, St. 

 Louis, and Kaskaskia, in the genera Forhesiocrinus, Taxocrinus, and 

 Onychocrinus, with a slight tendency in Taxocrinus of the Kaskaskia, 

 to reversion to the original form. Therefore this is not a case of 

 parallel development, but is that of the suppression of the earlier 

 structure, and its replacement by the later one, which disappeared 

 only with the extinction of the nonpinnulate division of the Flexibilia 

 toward the close of the Carboniferous. In the latest and most 

 extravagant genus of the group, Onychocrinus, it shows a tendency 

 to further development by the addition of another brachial, not 

 sporadically, but constantly, as a well-defined character among 

 species. From the tenacious grip that this structure had upon 

 several of the strongest genera, it must be regarded as a morphologi- 

 cal change of much importance, strongly affecting the phylogenetic 

 history of the group ; but yet subordinate, in my opinion, to the great 

 differentiation of the anal structure, and therefore not available for 

 defining large divisions. 



The second brachial modification is. marked by interesting 

 changes in the mode of branching of the rays above the first axillary 

 from a more or less regular division of the rays by successive bifur- 

 cations — dichotomy — to one into large main branches, or arm-trunks, 

 bearing ramules on one or both sides — heterotomy. Both were 

 established in the Silurian, and continued through the Devonian and 

 Carboniferous, and were in force, side by side, in the genera Taxo- 

 crinus and O?iychocrinus, at the close of the Subcarboniferous in the 

 Kaskaskia. The dichotomous plan, which was probably the primi- 

 tive one, was by far the most prevalent throughout ; and the heteroto- 

 mous plan was a modification which, while it ran parallel to the other 

 until the end of the group, did not supplant it. The differences 

 arising out of this modification afford very good characters for generic 

 distinction. 



