NO. 6 ECHINOID DISTRIBUTION AND HABITS — KIER, GRANT 39 



only four petals, all of which are approximately equal in length and 

 well depressed below the surface of the test. The tubercles are all 

 small, and the plastron is narrow. This species is distinguished from 

 the other large spatangoid that often occurs with it, Plagiobrissus 

 grandis, by its higher heavier test, lack of long spines on the upper 

 surface, more depressed petals, darker color, and wider plastron. 



Growth. — Two small spatangoids, 22 and 44 mm. long, seemed 

 different from all the other spatangoids known from the Keys. These 

 specimens differ from all other species in the length of their spines, 

 shape of test, arrangement and relative length of the petals, size of 

 peristome, and occurrence of fascioles. After considerable study, 

 however, these specimens were recognized as immature Meoma ven- 

 tricosa, unusual in that the growth changes that normally occur in 

 specimens much smaller than 22 mm. in other spatangoids are post- 

 poned in this species. Presumably it is because this species becomes 

 so large that even very young specimens are as large as adults of other 

 species. The young of this species would be large enough to be con- 

 sidered adults if no larger specimens were available. Therefore, to 

 clarify the relationships, these growth changes are described in detail 

 below. This information should make possible a reasonably accurate 

 prediction of the appearance of the young of other large spatangoids. 



Shape. — The shape of the test changes considerably during growth. 

 As the test increases in size it becomes relatively lower. The height of 

 the smallest specimen (pi. 9, fig. 1), 22 mm. long, is 67 percent of the 

 length, but only 53 percent in a specimen 71 mm. long (pi. 9, fig. 3), 

 and only 50 percent in a specimen 144 mm. long (pi. 9, fig. 4). The 

 ratio of the height to the length does not change, as shown by the 

 fairly straight line in the distribution of points in the scatter diagram 

 (fig. 8) . The relative width of the test is constant throughout growth ; 

 in the smallest test the width is 87 percent of the length, and in a 

 large specimen it is 88 percent (fig. 8). 



The petals are only slightly depressed in the two specimens 22 and 

 44 mm. long (pi. 9, figs. 1, 2) but in a specimen 71 mm. long (pi. 9, 

 fig. 3) they are considerably depressed, and in a full size specimen, 

 144 mm. long (pi. 9, fig. 4), they are deeply depressed. Likewise the 

 anterior ambulacral groove is shallow in the smallest specimen, 

 slightly more depressed in a specimen 71 mm. long, and deeply de- 

 pressed in a large specimen. Because of depression of the ambulacra, 

 the adapical interambulacra appear more inflated in the adults. 



The periproct on the smaller specimens is situated high on the pos- 

 terior truncation, and because this truncation is nearly vertical on 



