156 CRINOIDS. 



are great enough to render a separate generic term useful. The 

 chief point of distinction is the position of the anal opening, 

 which is placed at the end of a short ventral tube, instead of 

 being a simple aperture in the test, as in Agaricocrinus. It 

 seems, however, that much more importance has been placed 

 heretofore upon this structure in classification than it probably 

 deserves, as will be referred to later. 



Dorycrinus is directly traceable to a certain group of 

 Devonian crinoids, for which the name Gennseocrinus has 

 been proposed. The latter genus embraces a few small forms, 

 mostly from the Hamilton rocks. The species of Gennseocri- 

 nus (as for example O. cassedayi Lyon) are connected with the 

 Burlington and later Dorycrini by such forms as lately have 

 been found in the Kinderhook beds of central Iowa, and which 

 have been described as D. immaturus and J>. parvibasalis. 

 Dorycrinus, in combining the features of both, unites closely 

 the Batocrinoid and Agaricocrinoid groups. It agrees with 

 the first in the peculiar construction of the posterior side, in 

 the simple anal opening, and in the radial grouping of the 

 arms ; with the second in the shape and structure of the calyx, 

 and in the somewhat flattened distal portions of the arms, 

 approaching certain Eretmocrini in this respect. In the earlier, 

 more generalized forms, the close resemblance of Dorycrinus, 

 Agaricocrinus and Eretmocrinus or Batocrinus is far more 

 striking than in the later varieties which have become so 

 greatly differentiated. The most prominent features, perhaps, 

 to be noted in this connection are the monstrous ventral 

 spines, often reaching a length of three to five inches, as in 

 D. mississippiensis Eoemer, and D. roemeri M. & W. ; the 

 immense .basal expansion, as shown by D. missouriensis (Shu- 

 mard) and D. corniger us {Rsdl)-, and the stout heavy stalks 

 with large, conspicuous nodal joints. 



Actinocrinus is the type of a very remarkable group. 

 The earlier forms bear a close resemblance to those of Bato- 

 crinus, but the possession of only two plates in the second anal 

 tier serves readily to distinguish the two genera. As yet it has 

 not been found to occur below the Carboniferous. It early 



