54 SMITHSONIAN INSTITUTION 



stantly present in some species, as for instance in another and rarer British form, A. milleri 

 (Muller). They usually occur, finally, in groups of three or five. They are irregular in 

 form, and they resemble the anal plate in structure and mode of growth. 



Dr. Carpenter figures two clusters of these plates as seen at the inside of 



the calyx; * and J. S. Miller shows one such plate in each axil of his Comatula 



fimbriata, 2 which he calls " intercostal plates or joints." In my paper of 1906, 3 



after reciting the foregoing observations of Thomson and Carpenter, I said : 



These observations of the so-called interradial plates in Antedon have been rather dis- 

 credited by some subsequent authors in the course of controversial discussions, but I see no 

 reason for questioning them. The occurrences as described seem to me entirely in harmony 

 with the morphology of the group, and if they had not already been seen, I should confidently 

 expect them to be found by further research. 



Confirmation of this prediction can now be made in the most striking man- 

 ner both as to the comatulids and the stalked crinoids. Since W. B. Carpenter's 

 time the existence of a strongly plated interradial perisome extending down to 

 the radials has been shown among Recent stalked crinoids in the genera 

 Calamocrinus, Ptilocrinus, and others. As to the comatulids the latest state- 

 ment is by Clark in his Monograph of the Existing Crinoids, p. 339, as follows : 



In many of the recent comatulids more or less well-defined plates are found between the 

 division series and between the first two or three brachials of the free arms. These may be 

 comparatively small and distinct, or they may be large, forming a solid calcareous plating over 

 the perisome. They are most strongly developed in certain of the large very many armed 

 comasterids, as Comaster mitltifida, C. belli, C. typica and Comanthina schlegelii; and though 

 here restricted to small areas between the bases of the IBr x , are very prominent features of cer- 

 tain of the species of Antedon, especially of A. moroccana and A. diibenii. 



In young Comactinia these plates are well developed, from one to several, 

 subsequently resorbed. In Promachocrinus and Thaumatocrinus interradials 

 of a very distinct type occur resting on the basals, which apparently have noth- 

 ing to do with the perisomatic system, but are probably related in their origin 

 to the radials. 



It is evident from Dr. Carpenter's figure that these plates are more con- 

 spicuous on the interior of the young Antedon than at the exterior; in other 

 words, the growth of interbrachials is from within outwards. This accords 

 very well with the observed facts among the fossils of the present group. I 

 have many specimens showing how the interbrachial plates diminish in size and 

 number from the interior of the calyx to the exterior. In many cases where 

 they are well developed on the inside they appear as mere points at the outer 

 surface, and often they do not pass through the wall ; so that in a given inter- 

 radius we may count twice as many plates interiorly as exteriorly (PI. LXI, 



1 Philosophical Transactions Royal Society, London, 1866, pi. 33, fig. 7. 

 Natural History of the Crinoidea, 1 

 Journal of Geology, vol. 14, p. 508. 



a Natural History of the Crinoidea, 1821, Frontispiece', Fig. -G 



