NO. 6 ECHINOID DISTRIBUTION AND HABITS — KIER, GRANT 43 



in the larger with an angle of 135 degrees. The posterior petals are 

 less divergent, having an angle of 56 degrees in the smallest specimen 

 as opposed to 65 degrees in a large specimen. 



The petals are relatively much wider in the smaller specimens than 

 in the larger (see pi. 9). In the smallest specimen, 22 mm. long, the 

 width of one of the posterior petals is 8.6 percent of the length of the 

 test, but only 5.3 percent in a specimen 79 mm. long and only 4.2 per- 

 cent in the largest one studied, 155 mm. long. The greatest rate of 

 change (fig. 11) in the width of the petals occurs in the specimens 

 under 80 mm. in length. 



Phyllodes. — Phyllodes are well developed in the smallest specimen, 

 and do not change significantly during growth except by the addition 

 of pores. In the smallest specimen 6 pores occur in each of the porif- 

 erous zones of the anterior paired phyllodes, whereas 9-11 occur 

 there in specimens over 66 mm. long. Presumably these additional 

 pores were produced by enlargement and alteration of the small pores 

 in the ambulacral plates immediately adapical to the phyllodes. The 

 small sensory tubefeet that passed through these small pores must 

 have changed into the pencillate tubefeet found in the phyllodes. 



Peristome. — The outline of the peristome changes considerably 

 during growth. In the smallest specimen it is proportionately large, 

 with a length 22 percent of the length of the test, whereas in a large 

 specimen its length is only 15 percent of the test length. In the small- 

 est specimen (pi. 9, fig. 1) the peristomal opening is high, but as the 

 echinoid grows the labrum enlarges and extends anteriorly (pi. 9, 

 fig. 3) until in a large specimen (pi. 9, fig. 4) it extends nearly to the 

 anterior margin of the peristome. This development of the labrum is 

 also apparent in the profile views on the same plate. 



Periproct. — Besides changing its position, as described in the sec- 

 tion on shape, the relative size of the periproct changes during the 

 growth of the test. The periproct of the smallest specimen is pro- 

 portionately larger, with a height 23.8 percent of the length of the 

 test, whereas the periproct of the largest specimen is only 14.2 percent 

 of the test length. A scatter diagram (fig. 12) illustrates that the rate 

 of change in the relative size of the periproct opening is greatest in 

 the smaller specimens, with almost no change in rate in specimens 

 over 100 mm. long. 



Spines. — The spines are relatively much longer on smaller speci- 

 mens, particularly spines within the peripetalous fasciole (see pi. 10). 

 One measured spine is 5.1 mm. long in the smallest specimen, or 23 

 percent of the length of the test, whereas on a large specimen, one 



