566 



as opposed to the specific divisions already established. A 

 parallel case is found in the old practice of uniting all Monti- 

 culiporoids under the specific names of Clitetetes lycoperdon 

 and Stenopora fibrosa; other cases in the old methods of classi- 

 fying the now well established species of Fenestelhi and Pinna- 

 topora Vine (Glauconome of authors). This multiplication of 

 species by the splitting up of old landmarks of palaeontological 

 nomenclature, being the result of special studies, is the strongest 

 evidence of progression, showing at the same time how ex- 

 tremely limited our knowledge of nature really is. 



Five valid species were proposed by Prof. Hall in 1857, (Proc. 

 Am. Ass. Adv. Sci. Vol. 10), and in the following year (Geol. 

 Rep. Iowa, 1858), he proposed the name re versa for sinistral 

 examples of A. wortheni. To these the present work adds eleven 

 more, of which only two are from the Keokuk group, the re- 

 mainder being from the Chester. In proposing these species, I 

 fully appreciate the difficulties that the student will have to 

 meet in his attempts to identify them, yet, I can assure him, 

 that they are by no means insurmountable, nor any greater 

 than those which must be overcome in determining other groups 

 of organisms. The first thing to be done is to grasp the essen- 

 tial characters. These, of course, are surest determined by their 

 degree of prevalence and constancy, a test that can be applied 

 only when the material at hand is plentiful. During the course 

 of my work upon the genus I have had before me no less than 

 800 specimens, many of them nearly or quite perfect, this large 

 amount of material being contained partly in the State Museum 

 at Springfield, and partly in the extensive collections of 

 Bryozoa belonging to Mr. J. M. Nickles and myself. Some of 

 the species were represented by from 50 to considerably over a 

 hundred examples, two by only one each, 'while of the others 

 the numbers ranged from four to fifty. To determine the species 

 the variations in the following characters must be learned: (1) 

 the number of volutions made by the spiral axis in a given 

 space; this is generally very constant; (2) the outline and re- 

 lative diameter of the shaft and flange*; (3) when a portion or the 



* The term "flange" is applied to the projecting portion of the axis that would be 

 equivalent to the "thread" of a screw. By the "shaft" I mean the subcylindrical portion 

 of the axis about which the "flange" revolves. 



