28 CATALOGUE OF THE BLASTOIDEA. 



the fork-piece in a. Pentremites would cause it to be morphologically indistinguishable 

 from the radial of the Mesozoic Phyllocrinus, the type which was formerly supposed 

 to be aNeocomian Blastoid. The radials of this remarkable genus are closely similar 

 in shape to those of a Pentremites. The lip is occupied by an articular facet, the 

 lateral edges of which are produced upwards into two strong processes of varying 

 length in different species. These bound a deep sinus which lodged the different 

 arm-canals, surmounted by the ambulacral grooves radiating outwards from the 

 peristome. The relation of the ambulacra to the radials, therefore, was rather more 

 like that which they have in Stephanocrinus than the vertical position which they 

 take in Pentremites. But this fact shows conclusively that the radials of a Crinoid 

 and the fork-pieces of a Blastoid are homologous, plate for plate. It thus disproves 

 Billings's assertion to the contrary, and also the theory which Wachsmuth and 

 Springer x have expressed in the following terms : — " We may consider that the 

 ambulacrum is a recumbent arm ; the lower part of the forked plate up to the 

 ambulacrum is the first radial ... in Blastoidocrinus, the oldest-known Blastoid, the 

 suture is visible . . . that the two sides of the fork, instead of being interradial, form 

 together a second radial," &c. 



We have unfortunately been unable to examine any specimens of Blastoidocrinus, 

 which we only know from the figures of Billings and Schmidt. But we imagine the 

 suture referred to by Wachsmuth and Springer to be that between the radials and 

 deltoids, just as is shown in the hypothetical figure given by Billings 2 , in whose 

 interpretation of the calyx we entirely concur; and we have reason to believe 

 that in this respect the American palaeontologists are now in complete accordance 

 with us. 



E. The Deltoid Plates oe Lntekkadials. 



Interscapular plates, Say. 

 Second series of suprabasals, M'Coy. 

 Superior plates, Owen <$• Shumard, 

 Dcltoidstiicke, Roemer. 

 Interradials, De KonincJc. 

 Interradials, Shumard £f Yandell. 

 Second radials, Lyon. 

 Deltoids, Shumard. 



Interradials, Hall. 

 Interradials, Meek 4' Worthen. 

 Deltoids, Billings. 

 Deltoid pieces, Hanihach. 

 Ural plates, Etheridge, jun., § Carpenter. 

 Oral plates, Wachsmuth \ Barris. 

 Interradials, Montgomery. 

 Interradials, Wachsmuth 8[ Barris. 



From a purely morphological point of view the interradials are by far the most 

 interesting of the three series of plates which together make up the calyx of a 

 Blastoid. Basals and radials present themselves alike in every Crinoid, Urchin, 

 and Stellerid ; but the precise homologies in the unstalked Echinoderms of the 



1 •■ Revision of the Pakeocrinoidea. Part I." (Proc. Acad. Nat. Sci. Philad. L879, pt. 3), p. 13. 

 '-' '• Figures and Descriptions of Canadian Organic Remains,'" Geol. Survey Canada, 18o9, Decade iv. 

 p. 20, fig. 8. 



