380 PALEONTOLOGY OF ILLINOIS. 



ray and a sixth one in the middle. Sometimes these are all, or in part, 

 merely represented by nodes, or even in some instances nearly obsolete, 

 while in others they are extravagantly developed. Again, Dorycrinus 

 differs from both Actinocrinus and Amphoracrinus, in having, so far as 

 yet known, always two arms springing directly from each arm opening, and 

 these arms always simple. Our attention was first called to this by Mr. 

 WACHSMUTH, who is a very caretul and accurate observer, and we 

 found it to be so in all the specimens in his collection, while he assures 

 us that this is the case in all the specimens found by the various col- 

 lectors at Burlington, as well as all of those he has yet seen from other 

 localities, with the arms attached. Hence in all of the species of this 

 group described by Prof. HALL and others, where the number of arms has 

 been given merely from counting the arm openings and specimens of 

 these have since been observed with the arms attached their number is 

 found to be just double that stated in the descriptions. It is also worthy 

 of note that in this group the body plates are either plane, more or less 

 tumid, or tuberculiform, and never marked with proper radiating costse, 

 as we often see in Actinocrinus. 



The following is a list of the described species belonging 'to this 

 group, all of which are, so far as known, exclusively American types, 

 and confined to the Lower Carboniferous, viz : Dorycrinus Misptesigpi- 

 ensis, Roemer, and D. GouldiJ D. cornigerus,* D. divaricatus,* D. trino- 

 dus, D. quinquelobus,* I}, symmetricus,* D. desideratus, D. unispinuH* 

 and D. subaculeatus, Hall (sp.), all of which were described by Prof. 

 HALL under the name Actinocrinus. It likewise includes D. Missouri- 

 ensis* (=Act. Missouriensis, Shumard, ) and D. unicornis* ( =Act. nni- 

 cornis^ Owen and Shumard,) as well as our D. subturbinatus,* originally 

 described as an Actinocrinus. 



It might at a first glance be supposed also to include Actinocrinus 

 corniculus and A. brevis of Hall, but these forms (which Mr. WACHTS- 

 MUTH'S collections clearly show to be only varieties of one species) 

 have but a single arm from each arm opening (two to each ray), and 

 these arms, with the proportional stoutness, general structure, and 

 broad pieces at their bases, of Agaricocrinus. Hence this species can 

 only be regarded as a somewhat aberrant form of the latter genus. 



t Specimens of all the species marked with an asterisk have been found with the arms attached, and 

 presenting the characters mentioned above. The arms of the others remain unknown. 



} The proposed species A. tricornis and A. pendens, Hall, are believed to be only varieties of uni- 

 cornis of O. and S. 



