376 SMITHSONIAN INSTITUTION 



including species as late as the Lower Burlington, have a few interbrachials confined to the 

 lower part of the areas. A third set of species, beginning in the Kinderhook and including 

 all species from the Upper Burlington to the expiration of the genus in the Kaskaskia, have 

 the interbrachials strongly developed, rising well up along the rays and forming a concave 

 distal margin as already explained. The boundaries between the three sections are not 

 sharply defined, yet the general course of development is evident ; but in this respect it never, 

 in the Lower Carboniferous, went beyond the stage in which Forbesiocrinus began in the 

 early part of that period. While, therefore, there are in the later Keokuk and Warsaw species 

 of Taxocrinus with a profuse growth of interbrachials which have a superficial resemblance 

 to Forbesiocrinus, the manner in which these plates are disposed along the distal margin can 

 be relied upon to distinguish them. This is the case with species in which the structure of 

 the anal series is intermediate by reason of a strong growth of plates larger than the ordi- 

 nary perisomic plates at either side of the anal series — such as T. giddingei (PI. LIX, 



fig's- 3.5)- 



There are also Taxocrinoid forms having some anal plates attached to the right pos- 

 terior ray. These are often perplexing, and cause doubt to which family they should be 

 referred. If we could see the entire posterior basal in direct view we should probably find 

 in those cases also an approach to sutural connection at the distal face. They are intermediate 

 forms, in which their position must be determined by a slight preponderance of characters. 



It will be seen from the foregoing statements that while there was a progressive develop- 

 ment of the interbrachial system both in Taxocrinus and Forbesiocrinus, these changes were 

 not synchronous. Taxocrinus reached its acme in this respect in the earliest Lower Car- 

 boniferous, at about the same time that Forbesiocrinus first appeared in a very similar stage 

 of interbrachial development, and thereafter in the latter genus the growth of these parts 

 increased until the areas were completely filled. Thus Taxocrinus intermedins of the Kinder- 

 hook is in substantially the same stage as Forbesiocrinus nobilis of the approximately equiv- 

 alent Mountain limestone, and they can only be distinguished generically by the anal side ; 

 while all later Forbesiocrini can be readily distinguished from the later Taxocrini by the 

 interbrachials as well as by the other characters. 



In Onychocrinus the order of development of interbrachials was reversed, being pro- 

 fuse in the early stages followed by marked diminution toward the end. That genus is 

 fundamentally a Taxocrinus with heterotomous arms — the same modification that is seen 

 between the Inadunate forms Cyathocrinus and Barycrinus. 



The stem of Taxocrinus was of considerable length in the later Carboniferous species, 

 being much longer in T. collctti from the Keokuk than in T. communis from the Waverly. 

 Unfortunately the complete stems are so rarely preserved that no satisfactory comparison 

 can be made between the species generally. 



As the genus was formerly regarded by authors, Taxocrinus extended as far back as 

 the Ordovician, and included a variety of types the characters of which were not understood ; 

 later investigations have shown the necessity of distributing them in the genera Protaxo- 

 crinus, Meristocrinus, Gnorimocriniis and Eutaxocrinus. This leaves the parent genus a 

 compact, homogeneous group, beginning in the Middle Devonian, distinguished from all the 

 others by the absence of a radianal and the presence of three primibrachs. Only in the 

 latter character is there found a slight instability during the transition epoch of the Devonian, 

 when the evolution from two to three primibrachs as a constant character took place. The 

 genus culminated in the Lower Carboniferous, where it is represented by a number of vigor- 

 ous species ; and it continued to the end of that period. 



The nomenclatorial history of Taxocrinus is long, and is involved in a singular maze of 

 confusion. Although the most widely accepted genus of the Flexibilia, and one to which 

 more species have been referred by both European and American authors than to any other, 



