Mr. F. A. Bather on British Fossil Crinoids. 329 



penter tells me that he followed E. Billings ; but a reference 

 to Billings * shows that he spoke quite correctly of " azygos 

 interradials " or more correctly still " the plates of the 

 azygos interradius." With equal correctness one may speak 

 of an " azygos radial " ; but the plate in question although 

 originally a radial is certainly not the azygos radial, 

 for that is in the anterior radius. I shall therefore drop 

 the term " azygos " as either meaningless or misleading, 

 and shall call the plate the " Radianal " : this name 

 should satisfy those who regard the radial as derived from 

 the radianal, no less than those who prefer with me to 

 follow Carpenter. In the diagram and formulas I shall 

 designate it R' instead of Az. 



The next plate to be considered is the " special anal " 

 plate, hitherto designated x ; I shall retain this sign as 

 appropriately suggestive. Now, taken as proven the correct- 

 ness of Carpenter's views as to the radianal, we shall incline 

 to regard those forms as primitive in which the radianal is 

 more of a radial and less of an anal, in which it is not in an 

 asymmetrical position but corresponds to the other lower radial 

 plates. Such forms are Iocrinus (5), Heterocrinus (6), 

 Ectenocrinus (7), Anomalocrinus (8), and Merocrinus (11). 

 Now in all these forms x is supported by R and does not 

 touch R'. Obviously then x is not derived from R', but 

 originates above R, and on its left side. By parity of 

 reasoning we assume the next stage to be represented in such 

 forms as Hybocrinus (?) (3), Ottawacrinus (12), Dendrocrinus 

 (15), and Homocrinus (16), since in them R' is rather more 

 asymmetrical. In these X has passed down from above R 

 and now rests with its lower half between the right and left 

 posterior radials, being supported partly by R' and partly by 

 the basal. Carabocrinus (13), Botryocrinus (30), and similar 

 forms are, as all acknowledge, the next stages in the shifting 

 of the radianal; in these x has sunk still lower into the 

 dorsal cup and is now entirely in a line with the radials. The 

 great increase in size of x in some of the Botryocrinites and 

 Cyathocrinites does away with the need for a radianal, and it 

 consequently disappears ; this does not affect the argument. 

 In Parisocrinus (25) and Euspirocrinus (17) among pinnule- 

 less forms, and in the Poteriocrinites (26, 27), another change 

 has taken place ; the radianal has passed through a revolution 

 of 90°, and the lowest plate of the ventral sac (t) has sunk 

 down between R and x . Further than this it is needless to 

 follow the process ; this is enough to show that the course of 



* * On the Crinoidea? of the Lower Silurian Rocks of Canada, Canadian 

 Organic Remains,' Decade iv. pp. 23-24 et passim, Montreal, 1850. 



Ann. & May. A\ Hist. Ser. 6. Vol. v. 24 



