10 



other and readily distinguished by any one having any knowledge 

 of the structure of crinoids, but they are not linked by any series 

 of gradation with any known genus or species belonging to an 

 earlier age. 



Prof. James Hall described twelve ten-armed species from the 

 Burlington Group, viz.: A. bellitrema, A. con re.ru s, A. corrugatus, 

 A. brevis, A. excavatus, A. inflatus, A. geometricus, A. ornotrema, 

 A. pentagoiuis, A. planoconvexus, A. pyramid at us and A. stellatus. 

 Of these only eight, viz.: A. excavatus, A. brevis, A. pyramida- 

 tus. A. stellatus, A. couvexus, A. ornotrema, A. pentagonus, and 

 A. geometricus have been, in any manner, illustrated. We have 

 had occasion to examine the definitions and illustrations as well as 

 specimens belonging to these eight species and have found no 

 synonymy among them. A. bellitrema has been called a synonym 

 for A. ornotrema, and A. corrugatus has bsen called a synonym for 

 A. pyramidatus; but A. planoconvexus and A. inflatus, horn their 

 definitions, seem to be good species. It would be very gratifying 

 to have some one illustrate these two species, who has some 

 knowledge of fossils, in order that they might take their rank with 

 other described and illustrated forms. We described A. illinoisen- 

 sis, which has a depressed body with a peculiar pentagonal outline 

 and ten arms, from the Burlington Group. These are all the ten- 

 armed species that have been described from this Group. 



There have been described, from the Burlington Group, five 

 twelve-armed species, viz.: A. bullatus, A. fiscellus, A. nodosus, A. 

 Jwdgsoni and A. adamsensis, all of which have been illustrated, 

 except A. fiscellus. It was described by Prof. Hall as an Actinocrinus 

 and he said: "It is not of the type of Agaricocrinus, Troost, or 

 Amphoracrinusr His definition, however, would lead us to suppose 

 he had before him an Agaricocrinus and a distinct species. A. bul- 

 latus, Hall, and A. nodosus, Meek and Worthen, are distinct and 

 well characterized species very far removed from A. americanus, 

 with which they have been erroneously classed by some, as synonyms. 



The above are all the species which have been defined from the 

 Burlington Group, and, it will be noticed, that they possess either 

 ten or twelve arms. Prof. Meek mentioned the fact that he had seen 

 an eleven-armed specimen, but he did not describe it. 



There have been described from the Keokuk Group, three ten- 

 armed species, viz.: A. arcula, A. ivliitfieldi and A. wortheni. They 

 have been quite fully illustrated and are very distinct species. 



