291 Dr. F. A. Bather on the Homologies 



This is allied in structure to H. heros, Guer. j in both 

 species the sixth sternite of the male has four longitudinal 

 carina?, excluding the raised lateral margins, the outer pair 

 ending in a small spine and with a large tubercle at 

 the base, but the seventh sternite is broadly rounded at the 

 apex in titan and truncate in heros. I do not think that 

 titan is a mere colour variet3% though heros varies much in 

 colour, especially on the thorax ; but it may be a local 

 race. 



XXXIII. — The Homologies of the Anal Plate in Antedon. 

 By F. A. Bather, D.Sc, F.R.S. 



(Published by permission of the Trustees of the British Museum.) 



Of late years, since rigid distinctions were first drawn between 

 the various plates in the posterior interradius of Palaeozoic 

 Crinoids, the plate which appears, migrates, and disappears 

 in the posterior interradius of the larval Antedon, and is 

 called the anal plate, has been regarded as homologous with 

 the plate generally known as anal x (the brachiaual of 

 Bather, 1890) in the Crinoidea Inadunata and Flexibilia. 

 Dr. Austin Hobart Clark, however (1912, Journ. Washing- 

 ton Acad. Scr. ii. pp. 309-314, and 1915, Monogr. Existing 

 Crinoids, vol. i. part 1, pp. 331-339 *), attempts to prove 

 that the anal of Antedon is homologous with the radianal 

 (Bather, 1890). A plate presumed to be the same occurs in 

 the young of Promachocrinus. The representative of anal x 

 is found by Dr. Clark in the posterior one of the small inter- 

 radial plates occasionally observed in Antedon and other 

 normal comatulid genera, while in Promachocrinus he would 

 homologise it with one of the additional arm-bearing plates 

 (pararadials, Bather, 1900). 



Considering the extensive use that has been made of the 

 anal plates in the classification of the Palaeozoic Crinoids, 

 it seems advisable to examine Dr. Clark's arguments. But 

 first let us recapitulate the main characters of anal x and the 

 radianal, as seen in the Inadunata and Flexibilia. 



Both of these plates are intimately connected with the 



* The references, except when otherwise stated, are to the latter 

 work. 



