554 THE CEINOIDEA CAMERATA OF NORTH AMERICA. 



its culmination in the Upper Burlington limestone and Keokuk group, where 

 it became extinct, being the last survivor of the family. In Europe it is 

 represented at Tourney, Belgium, in the Yorkshire regions of England, and 

 at Waterford, Ireland ; but no trace of it has been found in the higher Car- 

 boniferous rocks of Scotland or Russia. 



Bemarks. — The genus ^c/inocrmts was made by the earlier writers on 

 Crinoids a receptacle for all — or nearly all — Camerata with a monocyclic 

 base, and in which an anal plate was introduced within the radial ring. This 

 accounts for the fact that the number of species referred to it reaches nearly 

 three hundred. The first departure from this rule was made by Austin in 

 1843, who introduced the genera Amphoracrinns and Periecliocrinus ; and 

 although his descriptions were meagre and partly incorrect, he gave well 

 known types for both forms, so that they could be readily identified. Owen 

 and Shumard followed in 1852 with Megistocrinus, and F. Roemer in 1854 

 with Dorycrinus. In the same year Casseday proposed the genus Batocrinus, 

 and in 1859, in company with Lyon, Eretmocrinus. But all these genera, 

 with the exception of Megistocrinus, were ignored by Hall, who from 1859 

 to 1861 described a great number of new species of this group. A more 

 important step toward a better understanding of this group was taken by 

 Meek and Worthen, who not only accepted the genera theretofore proposed, 

 but introduced three, new ones, viz., Steganocrinus and Strotocriniis in 1866, 

 and Plvjseiocrinus in 1869, which also are now generally accepted. After- 

 wards we proposed the genera Gmnwocrinus koA Tehiocrinus, and in 1881 the 

 number of species retained imder Actinocrinus, after deducting niimerous 

 synonyms, was reduced to less than fifty. But even these species were 

 susceptible of division into two sections, as already pointed out in 1866 by 

 Meek and Worthen, viz., 



A — species in which the higher brachials, sometimes from the second costals up, are 



grouped together, so as to form five protuberant lobes, from which tlie arms are 



given off from alternate sides. 

 B — species in which the arm bases are arranged in a continuous series around the calyx, 



i. e., the interbrachials are separated from the interambulacrals by the arm-bearing 



brachials. 



These differences we regard as amply sufficient for generic separation. 

 In addition to them there is a constant difference in the number of brachials 

 beyond the costals, — the higher orders of brachials in all species of section 

 A consisting of two or three plates, while each such order in those of sec- 



