BATOCEINID^. 487 



extremely long. First anal considerably longer than the radials, and much 

 narrower at the upper end ; it supports an elongate second anal, and at each 

 side a large interbrachial ; the succeeding plates are much smaller, and form 

 a more or less protuberant area, which extends to the posterior oral, and 

 contains the anus, which opens out laterally. Ventral disk conical or hemi- 

 spherical, the interradial spaces somewhat depressed. Oral plates large and 

 prominent ; the posterior one larger than the other four, and central, either 

 in contact with the latter, or separated from them by small perisomic plates. 

 The food grooves covered by superimposed interambulacrals, and toward the 

 margin of the tegmen by one or more large radial plates. Column long, 

 composed of large and smaller pieces ; the axial canal rather small and 

 pentalobate. 



Distribution. — Known only from the Mississippi Valley, where it occurs 

 in both Burlington beds, and also in the Keokuk group. 



Type of the genus : Agaricocriiius americanus. 



Hemarks. — This genus is most remarkable for the depression of the 

 dorsal cup, of which in the typical form only the arm facets and portions 

 of the interbrachials can be seen in a side view, the lower parts being flat or 

 more or less concave. Another good character is furnished by the distichals, 

 which, taking the form of arm plates, curve inward and interlock with one 

 another ; and last but not least, by the ponderous, gradually tapering, 

 biserial arms, and the elongate form of the interbrachials. In all of these 

 Agaricocrinus diifers essentially from Aorocrinus and Dori/crinus, with which 

 it has certain affinities. It has been most frequently confomided with 

 AmpJioracrimis, to which in some species it has a superficial resemblance in 

 the form of the calyx ; but the arm structure and position of the anus in the 

 two genera are different, and, in the typical forms, Amplioracrinus has only 

 two plates in the row above the anal plate. For specific separation we 

 rely upon the variations in the proportions of the interbrachial plates, and 

 the form and size of the costals and distichals, as the best distinctive char- 

 acters. The number and distribution of the arms, the form of the anal area, 

 and the condition of the oral plates, are also important. 



