MORPHOLOGICAL PART. 139 
sh) 
others not. In Lecanocrinus Billingsi* there is at the posterior side an anal 
«, together with a radianal; while at the other four sides the radials and 
costals of adjoining rays meet laterally.  Lecanocrinus macropetalus Angelin 
(not Hall),¢ 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 Tuxocrinus 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 z 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. 
Thaumatocrinus is the only recent genus which has a tube, such as we 
find among the Palxocrinoidea. 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 Rhodocrinidex. 
This seems to us a further proof that the plate a is not a primary element, 
but a supplementary plate, and was introduced only in cases where the 
structure of the anus required it. 
* Tconogr. Crin. Suec., Pl. XXII. Fig. 25. 
t Ibid., Pl. XIX. Fig. 4 
