10 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I44 



fig. X) have phyllodes with few pores, whereas species of Rhyncho- 

 lampas (chart 2, fig. W) and Echinolampas (chart 2, fig. Y) with 

 large tests, have phyllodes with many pores. It is significant that even 

 the young tests of large species have more pores than tests of the same 

 size of small species. 



PETALS 



There is a most striking evolutionary trend in the relationship be- 

 tween the length of the petals and the position of the margin. This 

 trend is illustrated in chart 4, in which all the important genera are 

 represented. The black area in each top and side view is the area be- 

 low the petals. In Middle Jurassic and Callovian genera the petals 

 are long and the margin abrupt, so that the area below the petals is 

 very small. By the Oxfordian the petals are still very long, but the 

 test is somewhat higher resulting in more area below the petals, par- 

 ticularly in side view. From this time onward there is a rapid increase 

 in the area below the petals due to a combination of shorter petals and 

 a higher test with steeper sides. As discussed in more detail on p. 21, 

 a cassiduloid probably could not burrow deeper than the lower limit of 

 its petals. Therefore, apparently the reason for this change is that 

 this increase in the area below the petals enabled the echinoid to 

 burrow deeper into the substratum. There are many advantages in 

 being able to burrow. That portion of the test under the sand would 

 be less vulnerable to damage by wave action, and there would be less 

 possibility of the test being tipped over. Furthermore, partially 

 buried echinoids would have more protection from predators, because 

 the test would be less visible and less exposed to the teeth or drill of 

 an adversary. Finally, the ability to burrow would greatly increase 

 the feeding area for the echinoid. When he was confined to the top 

 of the substratum, all his food came from the top surface and the area 

 within the substratum that could be reached by his phyllodal tube-feet. 

 Burrowing increased this area by the volume of the substratum 

 through which the animal was able to burrow. 



Besides the above evolutionary trend, there are several other changes 

 in the petals. In the Jurassic species the petals are normally very long, 

 extending to the margin of the test, and either narrow and almost 

 subpetaloid as in Galeropygus agariciformis (Wright), or very broad 

 and open with the outer pores greatly elongated and strongly con- 

 jugate as in Clypeus. The only exception to this generality is A^m- 

 clcoUtcs, in which the petals are shorter and narrower than in Clypeus. 

 In the Lower Cretaceous there are fewer species having the broad 



