14 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I45 



Arbacia improcera (Conrad) from the upper part of the Yorktown 

 formation (Late Miocene). Both species have the same shape, same 

 number of tubercles in the ambulacra and interambulacra, and same 

 number of ambulacral and interambulacral plates. A. crenulata dif- 

 fers in the surface ornamentation of the plates. In A. crenulata the 

 ornamentation consists of fine crenulations (pi. 1, fig. 5) that ex- 

 tend adorally, whereas in A. improcera there are granules (pi. 1, 

 fig. 6). Furthermore, in A. crenulata the naked areas in the ambu- 

 lacra and interambulacra extend farther adorally than in A. impro- 



< 



O 



< 



-J 



CD 



< 

 <r 



LU 



O 



OL 

 LU 

 CD 

 2 







+ + 



30- 





+ + + + 



+ 



co 







LJ20- 



+ + -h 





-j 



+ 





o 



+ 





DC 



+ 





UJ 



+ 





CO 







HlO- 







Fig. 7, 



10 20 30 40 



DIAMETER IN MILLIMETERS 



-Arbacia crenulata Kier, new species. Number of interambulacral 

 tubercles relative to the diameter of the test. 



cera. A. crenulata is easily distinguished from Arbacia waccamaw 

 Cooke by its much more ventral ambitus, lower interambulacral 

 plates, and smaller adapical tubercles. It is distinguished from Arbacia 

 rivuli Cooke in having fewer tubercles in the adapical interambu- 

 lacra. A. crenulata is similar to Arbacia sloani (Clark) from the 

 Late Miocene (Duplin marl) but unfortunately no well preserved 

 specimens are known of A. sloani, and it is not possible to make a 

 detailed comparison of the two species. 



Occurrence. — This species is most common in the "Buckingham" 

 facies and the barnacle-echinoid-oyster facies of the Tamiami forma- 

 tion. Very few specimens were collected from the typical Tamiami. 



Tamiami formation: Typical Tamiami : Loc. 12, 14. 



Tamiami formation, "Buckingham" facies: Loc. 20, 21, 22, 23, 

 24, 25. 



