1879/] NATURAL SCIENCES OF PHILADELPHIA. 247 



tinctions which we regard as important in the separation of families 

 and genera. On a former page we have alluded to the dome, which 

 we believe affords excellent characters for separation into families. 

 The general plan upon which it is constructed, whether rigid or 

 flexible, composed of movable or immovable plates; with large 

 oral plates, or covered with numerous small pieces; whether pro- 

 vided with a ventral sac; the location of the anus; all of these 

 in our opinion form good family distinctions. Next to the vault 

 must be considered the general construction of the calyx; the 

 elements of which it consists; whether it has a subbasal zone; the 

 presence or absence of interradials as a rule; whether the animal 

 was pedunculate or free floating; and last but not least, the struc- 

 ture and position of the respiratory organs. 



Among the best generic characters in these Crinoids, we find the 

 following: The general form of the body; the distribution and 

 arrangement of the different plates, both in the vault and in tlie 

 calyx, particularly the plates of the anal area and their propor- 

 tions ; the form and position of the apical dome plates, the posi- 

 tion of the anus and whether consisting of a proboscis or simple 

 opening ; the form of the column, the shape and proportional size 

 of its central passage; the construction of the arms and pinnulae. 



The arms and pinnulae of the Paheocrinoidea have not received 

 the attention which they deserve, at least not as to their generic 

 importance. A careful study of these organs, as they occur in 

 different genera, has convinced us, that not only the arms, but 

 also the pinnulae, in their variation, in their presence or absence, 

 afford generic characters. Only of late years has attention been 

 drawn to the ambulacral groove of the arm, when it was shown 

 from actual observation that in some genera the furrow is covered 

 by small plates alternately arranged. Meek and Worthen describe 

 the arms of Symbathocrinus as being covered by two rows of 

 single plates, arranged in alternation, and a similar structure has 

 been discovered to exist in the arms of Crotalocrinus and Enal- 

 locrinus. In Gupressocrinus, according to Schultze, the furrow 

 is braced over like a roof. In Cyathocrinus Iowensis, according 

 to Wachsmuth, and apparently in Gissocrinus, Angelin, the furrow 

 is covered by two rows of two successive plates, the plates of one 

 row alternating with those of the other. In Cyathocrinus longi- 

 manus, Angl. (Iconogr. Crin. PI. 2G, Figs. 4, 5), there are in place 

 of only two, a series of five successive plates from each side, 



