COMPSOMETRA SERRATA. 27 



In the youngest Pentacrinoids found (plate xin, figure 1) the basal 

 plates embrace the lower end of the oralia; the latter are distinctly concave 

 along their middle line, the edges being prominent. The infrabasalia can 

 no longer be seen distinctly, but on dissolving the calyx very carefully with 

 dilute hypochlorite of sodium they are seen lying inside the basal plates, 

 around the upper stalk-joints, where they are found also in the following 

 Pentacrinoid stages. There are still only 18 joints in the stalk. The first 

 (lowermost) 3 to 4 joints are very short, nearly spherical; the middle ones 

 are more elongate. In all of them the middle ring is very prominent. 



In the next stage, represented in plate xin, figure 2, the radials have 

 appeared; the anal plate, which appears shortly before the radials, lies 

 almost in the radial midline, the corresponding radial lying almost 

 in the middle between the oral and basal plates. The oral plates 

 are no longer embraced by the basal plates, and their lower end 

 is beginning to acquire an outward twist. A pair of small, new- 

 formed joints are seen at the upper end of the stalk, which is now 

 composed of 20 joints. 



In the stage represented in plate xn , figure 6, the oralia have 

 been separated from the basalia, so that a piece of naked skin is 

 seen between the adjoining ends of the two plates. The radial 

 plates have become large, with a prominent knob at the upper 

 edge, to which the costal is attached; at the upper end of the 

 costal the axillary has appeared. The small joints at the upper 

 end of the stalk have widened somewhat, but not grown in length. 

 Apparently no new joints have been formed. (The stalk is incom- FIQ. 5. - 

 plete in the specimens of this and the next stage.) ^ f f ^ 



The Pentacrinoid is especially characterized by its tentacles, tacrinoid of 

 which are long and slender and provided with calcareous spicules, 2STSS 

 thin, bow-shaped, finely thorny bodies, which are generally ing spicules. 

 arranged in a single series in each tentacle (figure 5). In specimens 

 mounted in Canada balsam these serially arranged spicules make a very 

 conspicuous and curious object. They are found equally well developed in 

 the tentacles of the grown specimens. 



The stage represented in plate xn, figure 7, is slightly more advanced; 

 the axillary is distinctly two-lobed and the costal is more elongate. The 

 anal plate is seen to encroach upon the oral; the lower part of the oral plate 

 has partly undergone absorption, and is partly covered by the anal plate; 

 also, the radial to the right of the anal plate is unsymmetrical, the side 

 joining the anal plate being less developed. 



In the stage represented in plate xm, figure 3, the arms have already 

 grown to some length and consist of about 5 to 6 joints; the exact number 

 of joints can not be made out distinctly because they are somewhat overlap- 

 ping and have no such prominent middle plate as the stalk-joints. The 



