5o8 FRANK SPRINGER 



ment. Hence it follows that, if in the growing Crinoid certain 

 spicules of the ventral perisome developed into well-defined plates, 

 which remained permanently in a definite position in the axils between 

 the radials or brachials, they would become the interbrachial (or 

 interradial) plates as we know them; so that whether a certain form 

 has interbrachials or not depends upon the extent to which the peri- 

 some developed downward into the axils. 



Some traces of the development of this element are to be found 

 in the Antedon larva. Sir Wyvill Thomson' thus describes their 

 occurrence : 



In one or two cases I have observed about the time of the first appearance 

 of the anal plate a series of five minute rounded plates, developed interradially 

 between the lower edges of the oral plates and the upper edges of the basals. 

 These interradial plates sometimes remain permanent in the mature Antedon 

 rosaceus, and they appear to be constantly present in some species, as for instance 

 in another and rarer British form, A. milleri. 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,^ which he calls "intercostal plates 

 or joints." These observations of the so-called interradial plates in 

 Antedon have been rather discredited 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. 



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

 more conspicuous on the interior of the young Antedon than at the 

 exterior; in other words, that the growth is from within outwards. 

 This accords very well w^ith' the observed facts among the fossils of 

 this 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 



1 Op. cit., p. 540. 



2 Philosophical Transactions, 1866, Plate 39, Fig. 7. 



3 Natural History of the Crinoidea, Frontispiece, Fig. 2, G. 



