28 THE VOYAGE OF H.M.S. CHALLENGER. 



British Museum. But the disc is sufficiently well preserved to show that the additional 

 ray is inserted between the two of the right side (D and E). 



The facts above mentioned may be usefully compared with similar variations which 

 have been noticed in other Echinoderms. In the only six-rayed Blastoid that I have 

 seen 1 there are but five ambulacra, though a pseudo-radial plate without a sinus is 

 intercalated between radials C and D, so that the dorsal surface of the calyx is very 

 regularly hexagonal. 



On the other hand Blastoids with only four ambulacra are more common ; but the 

 dorsal part of the calyx is more or less distinctly pentagonal, the fifth radial not being 

 incised for an ambulacrum. The two postero-lateral and the right antero-lateral one 

 (C, D, E) are the rays in which this modification has been noticed, C showing it twice 

 and the other two once each. 



Two tetraradiate examples of Encrinus liliiformis have recently been observed by 

 von Koenen ; 2 but it is curious that variations from the normal pentamerous symmetry 

 are rare among the Pelmatozoa, except in the genus Rhizocrinus. Four- and six-rayed 

 Urchins are not uncommon; while Ludwig 3 found half a dozen six-rayed individuals of 

 Cucumaria planci in a collection of one hundred and fifty. In all cases the sixth ray 

 was intercalated between the two forming the bivium, a fact which may be compared 

 with the absence of the middle ray of the trivium in the three Comatulaa with abnormally 

 interradial mouths mentioned above. 



1 See Etheridge and Carpenter, Catalogue of the Blastoidea in the Geological Department of the British Museum 

 (Natural History), London, 1886, pp. 40, 41. 



2 Beitrag zur Kenntniss der Crino'iden des Muscheikalks, Abhandl. d. k. Gesellsch. d. IViss. Gottingen, 1887, Bd. xxxiv. 

 p. 23 (of separate copy). 



3 Ueber Sechsstrahlige Holothurien, Zool. Anzeiger, 1886, Jahrg. ix. p. 476. 



