12 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



Gisle'n said that judged from the slight available information regarding undivided 

 arms in Antedon the indications are that there is here the same variability as in 

 Thaumatocrinus . 



In summing up Gisle'n said that if the first ramification is suppressed this process 

 occurs differently in different families, but with a certain regularity in closely related 

 forms. Usually it can then be demonstrated how the tendency to suppression affects 

 predominantly either the right or the left ray, which becomes a pinnule or disappears 

 altogether. If the arm is defective in its pinnulation the first pinnule and, beyond, the 

 first arm branch, appear as a rule on the same side of the arm as that on which the 

 suppressed first pinnule would have been according to the reconstruction. 



In the family Hyocrinidae usually both the suppressed and the lowest pinnule, 

 and in Calamocrinus the first arm ramification, are to the left. 



Among the pentacrinites Metacrinus has both the lowest pinnule and the first 

 arm branch to the right. 



In the Phrynocrinidae too little material is available to permit judging the position 

 of the first pinnule. 



The family Bathycrinidae may be divided into two natural groups. To one group 

 belongs Rhizocrinus, which has the first developed pinnule to the right on the eighth 

 brachial, though the suppressed first pinnule was to the left on the second brachial. 

 According to Gislen this type is the only exception to the rule that the lowest pinnule 

 appears on the same side of the arm as the suppressed first pinnule. In the other 

 group, exemplified by Bythocrinus, the suppressed first pinnule is usually to the right. 



Among the comatulids, in Eudiocrinus P c , the lowest pinnule, is on the left. 

 Where arms have been suppressed a similar tendency seemed also to show itself in a 

 few examples in the families Comasteridae and Uintacrinidae. An opposite tendency 

 is seen in Pentametrocrinus and Atopocrinus in which PI usually appears to the right. 

 In some young of certain species of Thalassometridae the right arms were often for a 

 time more weakly developed than the left. In the genus Thaumatocrinus the position 

 of PI is variable, and this is also the case in a few individuals of Antedon petasus with 

 undivided postradial series. Regenerates of this species pointed in the same direction. 



Gisle'n said that a connection, in the sense that the families having their lowest 

 pinnule developed in the same way are more closely related, can not be assumed if 

 there are no further facts forthcoming to support such a supposition. He remarked 

 that the Thalassometridae and certain Macrophreata agree in a number of features, 

 and it is probable that they are rather closely related. Possibly also, according to 

 Gisle'n, Thaumatocrinus is more closely related to the Antedoninae than it is to Penta- 

 metrocrinus. 



At least the majority of the comatulids presumably have not developed from forms 

 with undivided postradial series. It may be supposed that forms with undivided 

 postradial series arose in different comatulid families at different times, and are thus 

 only parallel types. The fact that in certain Zygometridae (Eudiocrinus) and in the 

 Comasteridae and Uintacrinidae the left arm branch is less strongly developed or 

 suppressed, and in certain Macrophreata and in some of the Thalassometridae it is the 

 right arm branch that is less strongly developed or suppressed seemed to Gisle'n to 

 support Kirk's assumption that the comatulids are of polyphyletic origin. 



