350 PROCEEDINGS OF THE ACADEMY OF [1890. 



the regular interradials, so as to isolate these from their fellows and 

 from the plates of the rays. 



The interradial plates, as stated before, are continued into the 

 vault, and in species in which there are but one or two bifurcations 

 in the calyx, this is quite readily ])erceived ; but in the more com- 

 plex forms the primary structure is frequently obscured by the intro- 

 duction of numerous supplementary jiieces, and it appears as if the 

 plates of the ventral side belonged to a distinct system. Looking at 

 a specimen of Strotocrinus, with its broad flanging rim, its hundred 

 and more arms crowded around it, and its thousands of minute vault 

 plates, decreasing in size outward, and in no way connected with the 

 interradials of the dorsal side, it is not surprising that Carpenter 

 regarded them, as we did at first, as structurally distinct from the 

 latter. 



To understand the structure of Strotocrinus let us refer to the al- 

 lied genus Steganocrinus, in which in a similar way the arms branch 

 off" alternately like pinnules from the two main divisions of the rays; 

 but while in Strotocrinus the lower part of the arms is incorporated 

 into the calyx, forming a continuous rim, from which the free arms 

 start off", in Steganocrinus the two divisions of the rays with their 

 small alternate arms are free, and extend out laterally in the form of 

 free tubular appendages. Now, it is very interesting to find that in 

 Steganocrinus the interradials meet the plates of the dome in such a 

 manner that it is absolutely impossible to draw a line between them 

 (see Steganocrinus pentagomis, Iowa Geol. Rep. Hall., Vol. I, Pt. II, 

 PI. 10, figs. 6 a, b.). The case of Steganocrinus becomes the more 

 instructive because this genus with its free arms may be regarded as 

 representing an early stage in the developmental history of Stroto- 

 crinus. 



A structure similar to that of Steganocrinus is found in all Cam- 

 erata in which the arms become free after the first bifurcation, and 

 from this condition all gradations can be traced to the complex 

 structure of Strotocrinus. We are therefore of the opinion that the 

 interradials interposed between the rays, and those at the dome, 

 must be regarded as parts of the same element, and as representing 

 a system of plates introduced between the actinal and abactinal sys- 

 tems, but actually belonging to neither. 



More than in the Actinocrinidae, Dr. Carpenter differs from us as 

 to the structure of the Platycrinidae. The ventral pavement of an 

 Actinocrinus he calls "a structure sui generis," i. e, different from 



