roteriocrinidte pentacrinus. 121 



Formation and Localities. 



The Oolite at Lansdovvn, near Bath ; at Dundry, near Bristol ; and at Farley Castle, 

 in the Forest marble, a subordinate member of the Lower Oolite. And, according to 

 Dr. Goldfuss, in the marly beds of the Jura formation at Baireuth, Boll, &c. 



This Pentacrinus when divested of its column, and lying with its rays fully expanded, 

 bears some faint resemblance to an Ophiura with branching arms, but when closely 

 examined, the comparison no longer holds good, for all the characteristics of a true 

 Pentacrinus at once become obvious. 



The body below the rays is small, and the lateral perisomic pieces fit closely into the 

 dorso-central plate, which resembles an enlarged columnar joint, with a star formed 

 depression on its articulating surface for the attachment of the column. See fig. 1 c, PI. 1 6. 



The five primary rays are short, and consist of two joints each, they then branch off 

 into a pair, each of which again bifurcate at some distance from the first divisions. 

 Other bifurcations take place until the final subdivisions amount to eighty or a hundred, 

 or even more. 



It should be observed that the rays of the Pentacnni divide in a different manner to 

 the Extracrini. In the latter it will be seen on reference to fig. 1 a, PI. 12, that after 

 the second bifurcations, the leading raj's send out lateral offsets from one side only, the 

 lateral branches being thinner than the rays from which they proceed. In the true 

 Pentacrines the arrangement is different, for where each ray divides, it branches into a 

 pair of equal magnitude as regards each other, but of diminished size as compared with 

 the primary rays, and whenever the bifurcations are repeated till the third or fourth sub- 

 divisions, it is always by a simple division of the ray into two smaller ones, both being 

 exactly equal in thickness. The rays in consequence of this arrangement bear some 

 resemblance to crinoids of the genus Poteriocrinus, and it has so happened that detached 

 fi"agments have been erroneously considered as belonging to the last named genus. 



The Pentacrines in the arrangement of the rays seem to constitute a link between the 

 Poteriocrines and Extracrines, the Pentacrinus Caput Medusa;, more particularly 

 approaching to the structure of the last named crinoids, and the P. tuberculatus, closely 

 resembling that of the Poteriocrines. 



The long and delicate tentacula are, owing to the shortness of the ray joints from which 

 they proceed, closely set ; they are also composed of short joints, and taper to a fine point. 



The pentangular column is long and slender, and apparently equal jointed throughout 

 its whole length, excepting where the newly formed joints near the summit were 

 continually in progress during the periodical growth of the animal. 



Q 



