128 PROCEEDINGS OF THE ACADEMY OP [188G. 



basals, radially when thej' are basals, and as they have in Hetero- 

 crinus and allied genera a radial position, we may assert that these 

 genera had no underbasals. The Heterocrinidae occupj'^, owing to 

 the absence of underbasals, relatively, the same position toward 

 the C^^athocrinidse and Poteriocrinidae as the Actinocrinidse and 

 Platycrinidse hold toward the Rhodocrinidse. They agree with 

 the Cyathocrinidse and Poteriocrinidae in having the azygous 

 interradius extended into a porous ventral sac. 



The arms consist of single joints, are comparatively strong, 

 and either bifurcate or remain simple from above the brachials. 

 The pinnules are given off not from alternate joints, but alter- 

 nately from each second, third or fourth joint. The}- are heavy 

 and long, and sometimes bifurcate near their tips. In the 

 typical form of Eeterocrinus alwa3S two joints are connected b}^ 

 syzygy, but we are unable to prove the syz_ygy in species referred 

 by us to Stenocrinus and Ohiocrinus, unless by the pinnules 

 which are given off regularly from .every third or fourth joint. 

 Most probably in locrinus and Stenocrinus the pinnules, and 

 sometimes even their branches, were developed into regular arms, 

 for the divisions follow each other alternately and at regular 

 intervals, and the branches have no further pinnules. 



The Heterocrinidae agree with the Hybocrinidae in the absence 

 of underbasals, but differ essentially in the proportions of the 

 calyx and ventral sac. In the former the calyx is extremely 

 small, the ventral tube large ; in the Hybocrinidae, on the con- 

 trary, the calyx comparatively large and the sac small. They 

 also differ in the structure of the arms, which in the Hetero- 

 crinidae are more highly developed. 



The structure of the ventral side has been observed only in 

 ^'Eeterocrinus juvenis,^^ where it is composed of five compara- 

 tively large plates, which enclose a small oral plate. The ventral 

 sac throughout the whole family is much stronger than in the 

 Hybocrinidae, reaching sometimes the tips of the arms, and it is 

 profusely perforated. 



Position, Locality, etc. Confined to the Lower Silurian of 

 America. The Heterocrinidae, although forming a neat little 

 group by themselves, may be considered as the forerunners of 

 several types which afterwards became distinct families. Eetero- 

 crinus foreshadows the Belemnocrinidae, locrinus the Poterio- 

 crinidae, while Anomalocrinus is a prophetic type for the Cyatho- 

 crinidae. 



