POTERIOCRINrD^.— EXTRACKDTUS. 95 



Genus 3. Extracrinus. (Amtin.) 





Definition. — Dorso-central plate composed of five pieces, (1); first series of lateral 

 plates, (2) ; second series five, (3). In mature and perfect specimens these latter are 

 jointed and prolonged downwards below the dorso-central pieces as represented in the 

 diagram, 2 a, 2 b, and 2 c; mouth centrical; column pentagonal. 



We have applied the term Extracrinus to designate this genus, deriving the term in 

 part from Extra, beyond, or without. The name is meant to refer to the second series 

 of perisomic pieces, which, in consequence of their singular development, extend 

 downwards much below the margins of the dorso-central plates. This peculiar form 

 of ossicula seems to be exclusively confined to the crinoids which we propose to 

 separate from the genus Pentacrinus, as they differ in many essential particulars from 

 the type of that genus. 



It is to be regretted that Mr. Miller in his arrangement of the Crinoidea has taken the 

 Pentacrhius Caput Medusce for the typical species, while at the same time his generic 

 plate represents the dissected skeleton of quite a different crinoid. In the hope to 

 remedy this intermingling of genera we propose to retain Miller's genus Pentacrinus, 

 and to continue the P. caput Medusce as the type of the genus, including in it all those 

 species of crinoids that answer to the generic standard, and which have inhabited the 

 seas during various geological epochs commencing with the liassic deposits, and des- 

 cending with various specific modifications to the seas of our own time. 



