Terms in Cnnoid 'Slorpliolorjy. 03 



Why should not all iiitcrradial plates below tho free arms, 

 except of course the basals, be called Interbraciiials, each row 

 being distinguished as first, second, third, &c. ? Tiius the 

 plates to which the term " luterradial " is now often wrongly 

 restricted would be called " the first Interbraciiials," or, when 

 they alone existed in the dorsal cup, simply "the Inter- 

 braciiials" {ihr). 



It would be convenient still to distinguish the corresjjonding 

 plates of the ventral surface as " Interambulacrals " {iamb). 



Interradial Plates. — Deltoids. 



In the genus Eusjn'rocrinus there occur on the oral surface 

 four cordiform or subtriangular plates. One of these is 

 situated in each interradius, except the posterior interradius, 

 and abuts on the U])turned portions of the radials, /. e. on the 

 radial processes. These four plates meet one another late- 

 rally, beneath the ambulacra, except in the posterior inter- 

 radius. In the posterior interradius there is a larger plate of 

 somewhat similar shape, which has an irregular surface. 

 This plate bears to the peristome the same relation as do the 

 four cordiform jdates ; it also partly sup])orts the ambulacra ; 

 it does not, however, bear the same relation to the radials, as 

 it is separated from them by a varying number of plates con- 

 nected with the anal tube. One or more of these latter plates, 

 on either side of the anal tube, meet the adjacent cordiforni 

 plate beneath the ambulacrum. 



The four cordiform plates are also met with in Gissocrinus^ 

 Cyathocrinus^ Carahoa-inus, Streptocrinus, and otiier Inadu- 

 liafe genera. The posterior plate with an irregular surface is 

 usually conspicuous in the same genera. The homologies of 

 these plates with plates occurring in the tegmen of other 

 Crinoids are still in dispute ; it is therefore advisable to give 

 to them some names that shall not have too great morpho- 

 logical significance. 



The posterior plate appears to have been perforated by one 

 or more pores, being in some cases quite cribriform *, and it 

 is probable that it subserved the functions of a madreporite 

 whatever those functions may be. It will therefore be con- 

 venient to apply to this plate the term " Madreporite/' wdiich 

 term, it should be remembered, has no strict morpholoo-ical 

 significance, since the position of the madreporite in other 

 Echinoderms is by no means constant. 



The four cordiform plates have often been regarded as orals 



* Wacbsmuth and Springer, " Perisomic Plates," loc. cit. p. 358. 



