MORPHOLOGICAL PART. I39 



others not. In Lecanocrinits Billingsi^ there is at the posterior side an anal 

 X, together with a radianal -, while at the other four sides the radials and 

 costals of adjoining rays meet laterally. Lecanocrimis macropetalus Angelin 

 (not Hall).t on the other hand, with exactly the same arrangement of anal 

 plates, has a large interbrachial plate at the four regular sides. The case is 

 even more perplexing in Taxocrinus Thiemei, of which some specimens have 

 one or three interbrachials, while others have none. We thus find within 

 the same genus, and even within the limits of the same species, interbrachials 

 present or absent, and according to Bather's theory the anal plates of one 

 specimen would be homologous with the anals of the Fistulata, and those of 

 the other structurally distinct. He seems to have regarded the anal plate 

 in the larva of Antedon as the homologue of the plate x in the Fistulata, 

 because the genus has no interbrachials. He says : " it is not an interradial ; 

 for the so-called ^ interradials ' that some observers claim to have seen are 

 only perisomic plates of no morphological importance ; further it is a most 

 gratuitous assumption to make Antedon the only form with an interradial in 

 the anal area, while devoid of true interradials in the other interradii." In 

 assuming that Antedon has no interradials, he employs the term in the 

 narrow sense in which it has been used heretofore • but since then we have 

 learned that all plates interposed between the rays and the ambulacra con- 

 stitute parts of the same element, and the same plates morphologically may 

 be interbrachial in one group, and partly or wholly interambulacral in 

 another. 



Thaicmato crimes is the only recent genus which has a tube, such as we 

 find among the Palaeocrinoidea. This tube rests upon a large interradial 

 plate, which, however, is not a special anal, for a similar plate is interposed 

 between the radials of the other four sides, exactly as in the Khodocrinidae. 

 This seems to us a further proof that the plate x is not a primary element, 

 but a supplementary plate, and was introduced only in cases where the 

 structure of the anus required it. 



* Iconogr. Crin, Suec, PI. XXII. Pig. 25. 

 t Ibid., PL XIX. Fig. 4. 



