1879.] NATURAL SCIENCES OF PHILADELPHIA. 287 



apparently in the Actinocrinidoe and Platycrinidae. This might 

 have induced us to place Helerocrinus and Iocrinus in a separate 

 division along with Hybocrinus and Anomalocrinus, which latter 

 two evidently belong to the same group, if we had not discovered 

 that the same diversity of structure apparently exists in species 

 of another family. In all Pentacrini, recent or fossil, that have 

 come under our observation, the cirrhi are radial and the ridges 

 interradial, with the exception of Pentacrinus Johnsoni, in which 

 according to Austin's figure (Rec. and Foss. Crin., PI. 15, Fig. i.) 

 the cirrhi are situated interradially as in Iocrinus. Owing to 

 this interesting coincidence, and also to the fact that in Dendro- 

 crinus, which in many respects closely resembles Helerocrinus 

 and its above-named associates, this columnar structure is the 

 same as in all typical genera, we have retained these genera 

 for the present in this family. We also retain those genera in 

 which the five underbasals are metamorphosed into three, as in 

 Gissocrinus, contrary to Angelin and Zittel, or into a single plate, 

 as in Stemmatocrinus, provided they otherwise agree with the 

 family. 



In the Cyathocrinidse, the anal area, though constructed of com- 

 paratively few plates, affords most excellent generic distinctions in 

 their arrangement. We note two principal forms; the one, which 

 we may call the Poteriocrinus form as it is best illustrated in 

 that genus is generally composed of three plates in the calyx, 

 which are arranged unsyrnmetrically and always directed toward 

 the right side of the body; the other, or Oyathocrinus anal 

 arrangement, with a bilateral symmetry, consists of a single plate, 

 which rests upon the truncate posterior basal. The former occurs 

 in connection with regular pinnulse along the arms; in the latter 

 the arm furrows are covered by small alternate pieces and the 

 pinnulse are wanting. 



There is scarcely any difficulty in referring all Cyathocrinidae, 

 from the Upper Silurian to the close of the Carboniferous, to one 

 of those two groups, though the anal plates vary in form more or 

 less in every genus. In genera from the Lower Silurian this 

 is more difficult. Yet there appears to exist even among the 

 latter and in connection with all others, an easy gradation which 

 indicates that both forms had very probably the same origin, and 

 that the later ones were gradually developed from the earlier 

 Silurian types. 



