98 POTERIOCRINID^ EXTRACRINl'S. 



of ihe abtloniinal pouch. Opposite to the divisions between tliese plates, are five 

 small but solid pointed pieces, whose points pass outward, and rest on the salient angle 

 of the pentagonal column ; abutting against and alternating with these smaller pieces 

 are the five jointed lateral pieces, which are also pointed, but in this case the points 

 pass further down and fit into the retiring angles of the column, but without being 

 attached to it. By this arrangement, a strong pentagonal foundation was formed for the 

 support of the numerous rays, and this elaborate structure was further strengthened by 

 a number of retractile fibres, or muscles, which also imparted mobility to the rays j five 

 of these muscles, more powerful than the rest, originated opposite to the centre of each 

 of the five larger perisomic pieces, which are inwardly notched for the partial reception 

 of these muscles, five smaller muscles passed upwards at the junctions of the larger pieces, 

 and were consequently attached to the prominent sides of the five lesser pprisomic pieces. 

 Five still smaller muscles originated at the outer angles of the lines of junction between 

 the five dorso-central pieces, altogether forming a complicated and wonderful combina- 

 tion of calcareous matter and muscular fibre, which must ha\ e possessed great strength 

 and power of resistance to external forces. Reference to PI. 12, c, d, e, will convey a 

 more perfect idea of the arrangemet of this portion of the animal than the mostlabouied 

 verbal description. 



Miller and other palaeontologists apjjcar to have been unacquainted with the arrange- 

 ment of the pieces forming the dorso-central plate and lower portion of the body of this 

 crinoid, for they have erroneously represented the five smaller lateral pieces as the pelvis, 

 and omitted to notice the five plates which form the true pelvis, but which are concealed 

 by the lateral pieces. 



These inaccuracies were no doubt caused by the difficulty of obtaining specimens 

 illustrative of the organism of this portion of the animal, for externally there is nothing 

 to indicate the presence of the small dorso-central pieces, and even up to the present 

 period very few specimens have been obtained sufficiently well preserved to exhibit 

 the great beauty of this contrivance, which at once combined strength with the required 

 amount of mobility. 



The Perisomic or Lateral Pieces. — The first series consist of five pieces ; 

 in the second series there are also five pieces, but as these are sub-divided in the adult 

 animal they frequently amount to twenty, a fact we believe hitherto unobserved. Each 

 of these consist of three pieces, (or in the older specimens of four), not soldered to- 

 gether, but articulating by radiating stria? and contractile fibres, which rendered them 

 capable of fiexion to a considerable degree, the lower one tapering to a point. These 

 pieces are prolonged externally downwards, the two lower joints extending over 

 several of the columnar joints, but to which they are in no way attached. 



