1870.] BILLINGS— ON CRINOIDEA AND BLASTOIDEA. 185 



Wyville Thompson; that it is doubtful whether or not the lancet 

 plates arc homologous with the radial plates of the Crinoids; and 

 that the ambulacra are more complicated in their structure than 

 is generally supposed. 



7. On the Structure of the genus Nucleocrinus. 



4. 



m y 



6. 



6 



Fi^. 3.— Apex of Xudeocrinus VerneiiiUi Troost. g, ambulacral groove : 

 J), pore throiig:h which groove enters into the interior ; s, one of the ten 

 spiracles; mv, oro-aual aperture. 4. Anterior side of a specimen; a, the 

 anterior interradial. 5 Apex of a specimen which has lost the integu- 

 ment that covered the centre. 6. Diagram of the plates of the test ; a, 

 amhulacral plate ; i, the hasals ; c, plates of the apex ; d, one of the 

 interradials ;/, forked plate. 



The body of this remarkable genus is ovate, elliptical or oblong, 

 and inclosed in a shell of strong perisomatic plates, which are, in 

 general, so closely anchylosed that the sutures between them 

 cannot be distinguished. According to Mr. Lyon, who, through 

 his long continued geological researches, has collected and studied 

 a vast number of specimens, there are three minute lozenge-shaped, 

 or quadrilateral basal plates, situated at the bottom of the 



YoL. Y. M Xo. 2. 



