J881.J NATURAL SCIENCES OF PHILADELPHIA. 215 



plates were joined by sj^zj'gies. The second pinnule, with but 

 four joints within the body, springs from the opposite side of the 

 fourth plate. The fifth plate, instead of a pinnule, supports a 

 regular arm, and the sixth again a pinnule, but from the same 

 side as the preceding pinnule. This is important as it suggests 

 the idea that the pinnule may have here developed into an arm. 

 That a transformation of this kind took place in some groups, is 

 more than probable, as will be shown presently. In Glyptocrinus 

 decadactylus only the second and third secondary radials are 

 joined b}^ s^'zygies, all succeeding pinnules being given off regularly. 



The number of arms has been considered of specific importance 

 among the Pal£eocrinid?e, and even genera have been based upon 

 this character. The greatest variation in the arm formula is found 

 among species of the tjpical Actinocrinidae, in which we include, 

 besides the genus Actijiocrinus, also Strotocrinus^ Teleiocrinus, 

 Physetocrinus and Steganocrinus. These genera agree in the 

 style of their ornamentation, and in the construction of the anal 

 area, which differs somewhat from that of all other Actinociinida?. 

 It is to be noted that in this group the specimens have, as a rule, 

 the same number of arms in the different rays, the few exceptions 

 being due to deficient or abnormal development of these parts. 



The genus Actinocrinus has been ver}- appropriately separated 

 by Meek and Worthen into two sections. The one, with Actino- 

 C7-inus prohoscidialis Hall, as a type, has the arm-bases arranged 

 in a continuous series all round ; the other, with Actinocr. midti- 

 radiaius as tj^pe, has the ra3S formed into more or less protuberant 

 lobes. 



A. proboscidialis of the first section, which represents the 

 simplest form of this interesting group, has only four arms to the 

 ray. The first departure is A. reticulatuii with four arms in all 

 but the two posterior rays, which have five ; the fifth arm where 

 it exists, being placed below the line of the others and appearing 

 somewhat crowded. Next in order are A. limahrachiatus and A. 

 clarus with five arms, in which one of the divisions in each ray 

 divides again. A. sexoTinatus and A. ojjusc^dus have six arms or 

 three to each division of the ray. A. muUihrachiatus and A. 

 penicillus have probably six, seven or eight arms, without regu- 

 larit}^ as to arrangement or distribution. A. ccelatus and A. spino- 

 (entaculus have a greater number than anj' other species of this 

 section, having normally eight arms to each ray. 



