182 Trans. Acad. Sci. of St. Louis. 



ceding pages I have pointed out the great deviations from the 

 pentamerous arrangement in the number and size of the in- 

 terbrachials, costals, and distichals, and in the number and 

 distribution of the arms. A natural classification rests upon 

 those prevailing characters which are most constant. This 

 fact points out at once the fallacy of adopting the interbrach- 

 ial plates, costals, distichals or arms as distinctive features. 

 The best characters for specific separation are (a) the general 

 aspect of the plates, (b) the external ornamentation of the 

 plates, and (c) the anal area. The geological formation is 

 also of value, as we may assume with perfect safety that fos- 

 sils found in the Chouteau Group belong to different species 

 from those found in the Burlington or Keokuk. Specimens 

 from the Burlington in general appearance look very different 

 from those of the Keokuk. Chouteau and Burlington speci- 

 mens have ten arms, a deviation from the number being rare. 

 In the Keokuk we see a tendency toward abnormal develop- 

 ment appearing in the number of arms. The regular Keokuk 

 species has twelve arms, which in itself is a departure from 

 the pentamerous arrangement. Deviations from that number 

 and in the distribution among the rays are very frequent as I 

 have pointed out before. Finding the pentamerous rule tol- 

 erably well preserved in the Chouteau and the Lower Burling- 

 ton Groups, with an increase in the amount of deviation from 

 it throughout the Upper Burlington and the Keokuk, proves 

 conclusively that the tendency was toward abnormal develop- 

 ment. 



Diagnoses of Species. 



Agaricocrinus Chouteauensis, Miller. 



Dorsal side flat or very little concave in the region of the 

 basal plates. Ventral side low, covered with small slightly 

 convex plates. Surface of the plates granular, rarely smooth. 

 Anal orifice small. — Geological formation. Chouteau. ^ 



Agaricocrinus brevis, Hall. 



Orals distinctly convex, the interambulacrals almost flat. 

 Posterior oral sharply conical. Plates of the dorsal cup be- 

 low the arm regions thickened, rising above the suture lines 



