110 rOTERTOCRINIDJi: PEXTACRINUS. 



Genus 4. Pentacrinus. (Miller.) 



(D,7>;^ 



Definition. — Dorso-central plate resembling an enlarged and thickened columnar 

 joint, (1), perisomic or lateral pieces five, (2), on which the first ray joints articulate. 

 Column generally pentagonal ; auxiliary side anns circular, and occurring at intervals 

 along the column in circles of five. 



The six pieces as represented in the generic illustration are all that constitute the body 

 of this animal below the rays. The organs of digestion are enclosed within a membraneous 

 pouch encircled by the rays. 



In describing this genus we have found it impossible to adopt the formula laid down 

 by Miller, because he has, in the recent species as well as in the fossils which we have 

 placed in the preceding genus, fallen into errors which are now quite obvious, and 

 although we may not be quite connect in our definition, for strange to say that owing 

 to the desiccated membrane which invests and covers the dorso-central plate within the 

 abdominal pouch adhering to, and concealing that portion of the calcareous skeleton, 

 the structure of the recent species as exhibited in those specimens of the P. Caput 

 Medusae hitherto brought to Europe, is not so well known to us as its fossil analogues, 

 yet we are enabled to point out the erroneous conclusions arrived at by Miller, and 

 reiterated by several succeeding naturalists. 



The dorso-central plate which Miller describes as " a pelvis of five joints," appears 

 to resemble an enlarged and thickened columnar joint, without divisions. The salient 

 angles of this piece are so placed as to appear between the retiring angles formed by 

 the junction of the perisomic pieces where they unite laterally to each other. If the 

 dorso-central plate is divided, the divisions are not perceptible, or it must be extremelv 

 small and altogether concealed as in the preceding genus, but of this we know nothing, 

 as all the fossil specimens of Pentacrinus as well as the recent species appear to 

 differ entirely in this respect from the Extracrini. 



