CBINOIDS. 151 



The Actinocrinoids are first known in the upper Silurian. 

 They early showed signs of departure from the primitive form, 

 and developed chiefly along two divergent lines. The one 

 group continued to the Burlington with, but slight tendencies 

 to modification in general structure; the other soon broke up 

 into a number of more or less well-marked sections, each of 

 which rapidly expanded into new generic types, until about 

 the close of the Keokuk, when, with a single exception, they 

 became extinct. In the present connection, therefore, mention 

 will be made of the following groups as comprising the Acti- 

 nocrinidsB: Periechocrinus, Megistocrinus, Amphoracrinus, 

 AUoprosallocrinus, Agaricocrinus, Dorycrinus, Gennseocrinus, 

 Eretmocrinus, Batocrinus, Actinocrinus, Teliocrinus, Physeto- 

 crinus, Strotocrinus and Steganocrinus. 



The general structure of the forms has already been 

 alluded to, but some minor anatomical points in various genera 

 may require further consideration. The first of the sections 

 above referred to includes only two types — Periechocrinus 

 and Megistocrinus. These genera differ from the other mem- 

 bers of the family chiefly in the relatively large calyx, rather 

 small branching arms, the large number of interradial plates, 

 and in the structure of the ventral surface. In Periechocrinus 

 the plates are smooth and thin ; in Megistocrinus rather thick 

 and more or less highly ornamented. The anal interradius has 

 three ossicles in the second tier, with many smaller pieces in 

 the succeeding rows. 



Amphoracrinus, in the general construction of the calyx, 

 closely approaches some forms of Agaricocrinus, but its arms 

 are very different, resembling those of the preceding group. 

 There are also other important distinctions. The earliest 

 Agaricocrinus appears in the Kinderhook. At the beginning 

 of the Keokuk a curious differentiation in some of the forms 

 took place, giving rise to AUoprosallocrinus, of which but a 

 single species is known as yet. The genus flrst mentioned is 

 characterized by a flattened or concave dorsal region in the 

 calyx, the free arms being given off low down on the margin 

 of the basal plane. The rays are somewhat separated, espe- 



