14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I44 



TUBERCULATION 



There is a trend in the cassiduloids for a development of a special- 

 ized adoral tuberculation differing from the adapical tuberculation. 

 In the Jurassic species, the adoral tuberculation is very similar to the 

 adapical, with tubercles of approximately the same size. By Neo- 

 comian time the adoral tubercles near the peristome are slightly 

 larger than the adapical tubercles. In the Cenomanian, the adoral 

 tubercles are considerably larger, and for the first time there is a 

 naked, granular, often pitted sternal area in interambulacrum 5. The 

 large adoral tubercles often have eccentrically situated bosses. In 

 the Late Cretaceous and Tertiary most of the genera have larger 

 tubercles adorally and many have naked sternal areas. The presence 

 or absence of this naked area seems of great significance taxonomi- 

 cally, and it is unfortunate that no Recent cassiduloids have been 

 studied in sufficient detail to learn its function. Probably the larger 

 adoral tubercles and the naked sternal area have a burrowing func- 

 tion. Nichols (1959, p. 417) describes the burrowing use of the 

 adoral spines in the spatangoids. 



SHAPE 



In the cassiduloids there is a trend toward elongation of the test 

 (chart 4). In many of the earliest genera, e.g., Clypeus and Mepyg- 

 urus, the test outhne is circular, and some are wider than long. By 

 the Early Cretaceous, and from that time until the present, most of 

 the genera are elongate. There are some exceptions such as some of 

 the Maestrichtian species of Hardouinia which are circular. Gen- 

 erally, the post-Jurassic species that have circular tests are highly 

 inflated, as in some of the larger species of Echinolampas, whereas 

 many of the circular Jurassic species are quite low. Presumably the 

 trend toward elongation reflects a change in living habits, probably 

 burrowing, where the elongate test would be better suited for moving 

 through the sediments of the sea bottom. 



SIZE 



There is no trend in the size of the test of echinoids. Some Tertiary 

 species are just as large as some Jurassic species. There do seem to 

 be more smaller species in the Upper Cretaceous and Early Tertiary, 

 with large numbers of small species in the Cassidulidae. 



