PALEONTOLOGY OF IOWA. 561 



" This crinoid diiFers so much in its form from the gene- 

 " rality of these fossils, that even without regard to the 

 " peculiar combination of its plates, its form alone would 

 " entitle it to rank as a new genus. The column being at- 

 " tached to the bottom of an inverted basin, and all the 

 " plates proceeding from the pelvis which forms the centre 

 " of this basin, running downwards, gives it the appearance, 

 " when a stem is attached to it, of a parasol or mushroom, 

 " (Agaricus, whence its name). The coronal integument 

 " which surrounds this basin is composed of hemispheric 

 " tubercles, and has the form of a cone or of a pentagonal 

 " pyramid." 



The above description does not fully express the character of the crinoid under 

 consideration; for an examination of the species described by Dr. TROOST shows a 

 basal series of three small plates with five radial series of three each, showing its 

 analogy with ACTINOCRINUS, as will be seen in the following formula : 



authority of Dr. TROOST. The name, proposed as new, occurs in the MS. of that author 

 in a " Monograph of the Crinoideae" transmitted to the Smithsonian Institution in 1850, 

 the typical species being Jlgaricocrinus tuberosus. Subsequently to that date, Dr. F. 

 RCEMER published in Bronn's Lethea Geognostica, Vol. ii, 1852 - 54, p. 250, the same 

 fossil, refering it to the Genus AMPHORACRINUS, as A. americanus. This genus was sepa- 

 rated from ACTINOCRINUS by AUSTIN in 1848; and if these differences be admitted as of 

 generic importance (which I am not disposed to do), there are still stronger reasons for 

 separating the Jlgaricocrinus, which differs quite as strongly from the typical form of 

 Jlmphoracrinus as that does from Jlctinocrinus. For this reason I retain the name Jlga- 

 ricocrinus for the species under consideration, as well as for that designated A. tuberosus 

 by Dr. TROOST. 



I have before me at the present time, in the collection of the late Dr. TROOST and 

 from other sources, specimens which indicate the existence of half a dozen species of this 

 genus. I have already described four species; leaving the indication of the others till I 

 have ascertained more fully to what extent variations of form, relations, and proportions 

 of the plates of the body, may occur in the same species. 



[ IOWA SURVEY.] 71 



