VERTEBRAL COLUMN. 327 



Figs. 1, 2 ; PI. 52. Fig. 3.; and PI. 53.) will give a more 

 accurate idea than can be conveyed by verbal descriptions.* 



Vertebral Column. 



The upper part of the vertebral column of Pentacrinites 

 is constructed on principles analogous to those already de- 

 scribed in the upper part of the column, of the Encrinite.f 



All the joints of the column, when seen transversely, 

 present various modifications of pentagonal star-like forms ; 

 hence their name of Asteriae, or star-stones. 



These transverse surfaces are variously covered with a 



* PI. 51 represents a single specimen of Briarean Pentacrinitc, wiiich 

 stands in iiigh relief upon tiie surface of a slab of Lias, from Lyme Regis, 

 almost entirely made up of a mass of other individuals of the same species. 

 The arms and fingers are considerably expanded towards the position they 

 would assume in searching for food. The side-arms remain attached to the 

 upper portion only of the vertebral column. 



At PI. 53. Fig. 1 and 2 represent two other specimens of the same spe- 

 cies, rising in beautiful relief from a slab, which is composed of a congeries 

 of fragments of similar individuals. The columns of these specimens, Fig. 

 2, a, show the side-arms rising in their natural position from the grooves 

 between the angular projections of the Pentagonal stem. At PI. 53, Fig. 1. 



¥. F. are seen the costal plates surrounding the cavity of the body ; at H^ 

 the Scapulse, with the arms and fingers proceeding from them to the extremi- 

 ties of the tenlacula. 



At PI. 53. Fig. 3. exhibits the side-arms rising from the lower part of a 

 vertebral column, and entirely covering it. Fig. 4. is another column, on 

 which, the side-arms being removed, we see the grooves wherein they arti- 

 culated with the alternate vertebra). Fig. 5. exhibits a portion of another 

 column slightly contorted. 



•j- The columnar joints of the Briarean Pentacrinite are disposed in pieces 

 alternately thicker and thinner, with a third and still thinner joint inter- 

 posed between every one of thera. PI. 53. Figs. 8, and 8», a. b. c. The 

 edges of this thinnest joint appear externally only at the angles of the column; 

 internally they enlarge themselves into a kind of intervertebral collar, c. 

 c. c. 



A similar alternation in joints of the Pentacrinites sub-angularis is repre- 

 sented in PI, 52. Figs. 4 and 5. 



