190 PALAEONTOLOGY OF ILLINOIS. 



Section b. — Species with a subccntral proboscis, bearing the opening at its upper 

 extremity. 

 Stroiocrinus erodus, S. liratus, S. umbrosus, S. subwnbrosus, S. segilops and 

 S. glyptus— (Actinocrinus (Calathocrinus) erodus, A. liratvs, etc. Hall), 



The differences between the physiognomy of the species of this group, and 

 that of the types upon which the genus Actinocrinus was founded, are so strik- 

 ing as to attract the attention of the most casual observer, and render their 

 separation easy, even at a glance, where the entire body can be seen. We 

 therefore regard them as forming a natural group, that should be separated 

 generically from Actinocrinus* particularly as it is now known that that group, 

 as usually defined, would include a very large number of species in this country, 

 presenting a great diversity of forms, and departing often widely from the 

 original type in various ways. It has usually been the practice of naturalists 

 to separate into distinct genera, upon less sharply defined characters, the species 

 of a very large group, than would be done in cases where the species are less 

 numerous. Although this may not, in all cases, be philosophical, it is certainly 

 convenient, and is, as we believe in the present instance, required by a correct 

 classification of these extinct forms. 



Accustomed, as palaeontologists have most generally been, to distinguish the 

 genera of crinoids almost entirely by the number and arrangement of the pieces 

 composing that part of the body below the arm-bases, without regard in many 

 cases to the most marked differences in other parts, it is not probable that the 

 genus under consideration will be at once generally accepted, but that the pro- 

 priety of such a separation will be ultimately admitted, we have no doubt. 



The transition from this genus to Actinocrinus, is through some sections of 

 the Batocrinus group, as we have defined it; and in some cases, where the 

 margin of the expanded summit has been evenly broken away, as represented 

 in the figures of Strotocrinus (Actinocrinus) umbrosus, Hall, in the Iowa Report, 

 pi. 11, these forms might be confounded generically with such species of the 

 Batocrinus group as Actinocrinus sequalis, Hall, represented by fig. 4 of the 

 same plate. It will be observed, however, that even in this case there are 

 characters by which these types can be distinguished by a careful observer. 



In the first place, the species of the group under consideration, always have 

 the body below the horizon of the arms, more elongated in proportion to the 

 height of the vault than in Batocrinus. Again, in Batocrinus the body plates 

 are scarcely ever marked with radiating costae, as we usually see in Strotocri- 

 nus. Another readily observed distinction is, that in the Batocrinus group the 

 second primary radial pieces, are almost, if not quite, invariably quadrangular, 

 and generally wider than high, instead of hexagonal, as in Strotocrinus. This 



* Prof. Hall first proposed to separa'e this type as a subgenus under Actinocrinus, 

 but used for it the preoccupied name Calathocrinus. 



