50 



Stone Lilies of the Trenton Limestone. 



receptacle, called the pelvis, supporting the viscera and body. From the- 

 upper margin of the pelvis there arise five upright rows of other plates^ 

 called the rays, which constitute a large portion of the sides of the cup. 

 When spread out upon a flat surface, in their natural order, these radiate 

 from the centre, in the form of a star, and hence the' crinoidca are pro- 

 perly considered to fall within the department of the Radiata. 



In many species these rays are divided into numerous branches, but i» 

 others they remain single to their extremities. In the branched varieties, the 

 five undivided portions are called the Primary Rays, and in many species- 

 these consist of three flat plates, each as seen in the figures 1 and 2. — 

 Above the Primaries follow the Secoxdary, Tertiary, Quaternary, or 

 Quinary Rays. At a variable distance from the base of the body or cup of 

 the Criuoid, the rays become free, or no longer form a part of the general 

 covering of the animal. They are then all called by the common name of 

 ARMS, no matter whether they consist of Secondary, Tertiary, or Quater- 

 nary Radials. This liberation of the rays from the walls of the body some- 

 times takes place near the base, and then even a portion of the Primary rays 

 is included in the arms, but in other genera they do not become free until the- 

 third, fourth, or fifth division- 



Fig. 2" is an encrinite of the genus Glyp- 

 TOCRiNus, with the branching arms above, and 

 a short piece of the staJk, at the base. The. 

 figure does not represent any particular speci- 

 men, but was drawn to give an idea of tha 

 cup-like body, with its external skeleton of an- 

 gular plates and branching arms. It will be 

 observed that the rays, two of which only caa 

 be seen in this figure, originate in the base of 

 the body, and proceeding upwards, are blended 

 in the general covering of the animal, until at 

 length they become free at the top and con- 

 stitute the arms. Hence the arms of a Crin- 

 oid are simply coutmuationa of the rays. 



Fig. 2, 



These organs constitute the Radial System of the Crinoids, and can be 

 detected, though often umler an extremely modified form, in all the species 

 vet known. In those of the most simple organization, there is little else 

 t^ be seen— the rays forming the wiiolc of the skeleton, but in others the 

 top of the body,"^ or the abdomen, is eoTcred over by numerous other 

 plates, the abdominal plates, which constitute a firm, dome-shaped roof. 

 Caiino- all the space between the free arms. In the species now living m tho 



