INTERRELATIONSHIPS 73 



of the rays, producing the 3 IBr structure, cannot be explained in any such 

 way, and the occurrence of this form in the Silurian is so limited and excep- 

 tional that it may scarcely be said to have had a beginning before the Devonian. 

 It is clearly the successor of the first one in the Paleozoic. The 2 IBr structure 

 continued from the Ordovician through the Devonian and Carboniferous, with 

 constantly diminishing importance. The 3 IBr structure, on the other hand, 

 having barely made a beginning in the Silurian, shows a steady increase 

 through the Devonian and Carboniferous, and prevailed exclusively in the 

 Warsaw, St. Louis, and Kaskaskia in the genera Forbesiocrinus, Taxocrinus, 

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

 reversion to the original form. 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 morphological 

 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 bifurcations — 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 Taxocrinus and Onychocrinus at the close of the Subcarboniferous 

 in the Kaskaskia. The dichotomous plan, which was probably the primitive 

 one, was by far the most prevalent throughout; and the heterotomous plan was 

 a modification which, while it ran parallel to the other to the end of the group, 

 did not supplant it. The differences arising out of this modification afford very 

 good characters for generic distinction. 



d. The modifications in the plates of the interbrachial areas might properly 

 have been considered in connection with those of the anal side, inasmuch as 

 they all belong to the system of supplementary plates as distinguished from 

 that to which the brachials belong. Sir Wyville Thomson was led by his re- 

 searches on the embryology of Antedon 1 to regard the skeleton of the crinoid 

 as composed of two systems of plates, which he states to be thoroughly dis- 

 tinct in their structure and mode of growth. These he designated as the 

 Radial, and the Perisomatic, systems of plates. The former is distinguished 

 by being chiefly made up of a peculiar fasciculated tissue of parallel rods, while 

 the latter commence as simple cribriform films imbedded in the outer layer of 



1 Philosophical Transactions Roy. Soc. London, 1865, p. 540. 



