166 MESOZOIC AND CENOZOIC ECHINODEEMATA OF THE UNITED STATES. 



Indian 1:0.62 or 1:0.63. A. merrilli also differs from A. alius in having its peristome more 

 angular and less oblique and its periproct more pointed posteriorly and farther from the pos- 

 terior margin. It is nearer A. americanus but is less depressed and has a more subcorneal upper 

 surface. 



Localities. — Kendrick lime quarries near Ocala, Marion County (type), and Clarke, Alachua 

 County, Fla. 



Geologic horizon. — Ocala limestone of Vicksburg group, lower Oligocene. 



Collection.— U . S. National Museum (164934, A; 137870, B). 



Genus OLIGOPYGUS De Loriol (amended). 



Oligopygus was founded in 1887 by De Loriol on the species Oligopygus wetherbyi from 

 the Tertiary of Florida and so far as known has few representatives outside of the United 

 States. De Loriol possessed but a few specimens and, although his diagnosis is well drawn, 

 the large number of specimens studied by the writer (over sixty) has shown the necessity of 

 amending the description of the genus in a number of important particulars, of which the most 

 important is noting the fact that it includes forms in which the periproct is inframarginal. 



Test moderately large, with thick walls, especially at the ends; usually oval, may be 

 subcircular, or subpentagonal; sides tumid; under surface slightly concave, with a deep, trans- 

 verse depression around the peristome; upper surface convex, more or less depressed; apex 

 central or anteriorly excentric. 



Apical system central or subcentral, compact, small, with four perforated basal plates; the 

 five radial plates small, with minute pores; madreporite central, tumid, extending to or between 

 the posterior radial plates. 



Ambulacral areas broadest at the ambitus, the anterior paired areas curving posteriorly 

 near the peristome; faintly tumid on the upper surface; dorsal portions varying from sub- 

 petaloid to petaloid, the petals differing"' slightly in length and more or less open, otherwise 

 similar. Poriferous zones broad; pores equal or nearly so, pairs united by a long groove. 

 Beyond the petaloid portions to and beyond the ambitus each plate is perforated at its outer 

 end by two or three pairs of very small pores. 



Peristome central, or subcentral, pentagonal, slightly elongate transversely, at the bottom 

 of the transverse depression in the under surface; floscelle and jaws wanting. Periproct very 

 small, circular, opening on the inferior face between the peristome and the posterior margin; 

 may be inframarginal. Tubercles small, equal, more or less scattered, surrounded by deep 

 scrobicules and imperforate. 



Oligopygus wethekbyi De Loriol. 



Plate LXXVIII, figures 2a-d, 3a-b. 



Oligopygus wetherbyi De Loriol, 1887, Recueil zool. Suisse, vol. 4, No. 3, pp. 396-398, PL XVII, figs. 7-8. 

 Oligopygus wetherbyi Stefanini, 1911, Soc. geol. italiana Boll., vol. 30, p. 705. 



I Stefanini, 1911, Revista italiana di paleontologia, vol. 17, pt. 2, p. 11. 



Determinative characters. — Test with thick walls, oval, sides tumid, upper surface depressed 

 and rather flat on top; under surface with a very deep, long, flaring, transverse depression around 

 the peristome. Ambulacral areas wide at the ambitus; dorsal portions petaloid, constric- 

 tions at the ends of the petals somewhat pronounced. Apex and apical system central or 

 subcentral. Peristome pentagonal, opening at the bottom of the depression in the under sur- 

 face. Periproct very small, circular, situated about midway between the peristome and pos- 

 terior margin. 



Dimensions. — Specimen A: Length 34 millimeters; width 29 millimeters; height 16 milli- 

 meters. 



Description. — The writer has not had the opportunity of seeing the type of this species, 

 but De Loriol's full and clear description and excellent drawings have permitted its ready iden- 

 tification among the material studied. The test is unusually thick, especially at the ends; 



