12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 45 



56 in specimen 30 mm in diameter; the number of primary tubercles 

 in each ambulacrum varies from 7 in a specimen 1 1 .8 mm in diame- 

 ter to 20 in a specimen 33 mm in diameter; one large pit in each 

 ambulacrum near peristome (pi. 1, fig. 4) ; primary tubercles very 

 large adorally, but greatly reduced in size and number to ambitus. 



Inter ambulacra. — Plates low, 22 in interambulacrum of specimen 

 13.8 mm in diameter, 24 in specimen 19.7 mm in diameter, 28 in 

 specimen 30 mm in diameter ; primary tubercles very small in area 

 extending from apical system to slightly above ambitus, no tubercles 

 in median region, one tubercle on each plate near adradial suture, in 

 some specimens tubercles smaller on every other plate in some series ; 

 tubercles very large in area from slightly above ambitus to peristome ; 

 usually two tubercles on each plate. 



Fig. 4. — Arbacia crenulata Kier, new species : Apical system of holotype, 

 U.S.N.M. 648133, from the "Buckingham fades" of the Tamiami formation, 

 from loc. 20, X 4. 



Peristome. — Very large, one-half as wide as horizontal diameter 

 of test, pentagonal, relative size of peristome constant throughout 

 adult growth (text fig. 5) ; gill slits wide, continuing considerable 

 distance on surface of test (pi. 1, fig. 4) ; auricles high, slender, not 

 joined. 



Periproct. — Opening elongated along line passing through inter- 

 ambulacra 1 and 3; size constant throughout growth (text fig. 6). 



Tuberculation. — All primary tubercles imperforate, smooth, on 

 highly inflated bosses ; surface of all plates, where tubercles do not 

 occur, crenulated with series of narrow grooves and ridges running 

 from apical system to peristome (pi. 1, fig. 5). Number of tubercles 

 relative to size constant throughout growth (text fig. 7). 



Comparison with other species. — This species is very similar to 



