INTERRELATIONSHIPS Jl 



" anal " and the concavity becomes straightened out and disappears, the " anal " concurrently 

 being shoved diagonally forward (toward the left) and disappearing by resorption. 



After noting that in certain fossil groups there may be traced a progres- 

 sive variation in the position of the radianal from a primitive position under 

 the right posterior radial to an oblique position under the lower left-hand cor- 

 ner of the radial, and finally to complete elimination, as shown by my paper of 

 1906, he continues: 



The position of the so-called " anal " in the larvae of Promachocrinus, lying within a 

 concavity in the lower left hand portion of the radial to the right of the posterior interradius, 

 and its migration upward and toward the left, leave no room for doubt that the so-called 

 anal of the pentacrinoid larvae is nothing more nor less than the radianal of the, fossil 

 forms (p. 332). 



He then discusses the case of Thaumatocrinus renovatus P. H. Carpen- 

 ter, with its anal tube and a vertical series of plates to the left of it arising 

 from an interradial in the posterior interradius (PL A, fig. ga herein), and 

 shows that Carpenter's specimen was the young stage of the normally 10-rayed 

 Promachocrinus abyssorum ; that the series of plates above mentioned repre- 

 sents an arm similar to what he (Clark) has observed in some thirty 6-rayed 

 specimens. His observations on this point are as follows (p. 338) : 



The interesting Thaumatocrinus renovatus is the young of the species later described as 

 Promachocrinus abyssorum (with which it was found associated) just after the resorption 

 of the radianal and the formation of all of the interradials from which the five additional 

 arms are commencing to grow. The posterior interradial arm as seen in the so-called 

 Thaumatocrinus is the first to form, and is consequently larger than the others ; but from 

 the size of this posterior arm and the breadth of the interradials I suspect that smaller arms 

 borne on the other interradials have been lost, as these interradial arms when small are 

 extremely delicate. During growth the posterior interradial arm of Thaumatocrinus becomes 

 reduplicated on all the other interradial plates, and all of the five interradial arms gradually 

 increase to the size of the five primary arms (the extensive plating of the disk at the same 

 time disappearing by resorption) so that the 10-armed Promachocrinus abyssorum results. 



Anal x in the fossil forms may be reduplicated in the form of a series of interradials, 

 one in each of the interradial areas, and therefore, bearing in mind the greater perfection of 

 the radial symmetry in the recent types, it does not surprise us to see the same thing in the 

 recent comatulids. In some thirty 6-rayed specimens which I have studied the supernumerary 

 ray is in all cases but two inserted behind the left posterior — that is, between the two posterior 

 radials and receiving its ambulacra from the groove trunk to the left. It is impossible to 

 interpret this otherwise than as the persistence and subsequent development of anal x in 

 types in which the interradials, including anal x, are normally resorbed immediately after 

 formation, exactly as it is developed in Promachocrinus and Thaumatocrinus. 



It is important to note, with reference to the development of the so-called 



anal plate in another family already alluded to and hereinafter to be described 



in detail, that according to all the foregoing statements the plate in Antedon is 



not developed until after the radials have formed and the primibrachs have 



begun to appear. 



