OLIGOCENE ECHINODERMATA. 167 



quite regularly oval in marginal outline, though sometimes narrowing anteriorly; margin rounded 

 and inflated, the anterior end somewhat more so than the posterior, giving that end a bulging 

 appearance forward. The upper surface is uniformly convex; but depressed and rather flat on 

 top; under surface slightly concave near the middle and with a very deep, transverse depression 

 around the peristome. The apex is subcentral, or slightly forward of the center. 



The ambulacral areas are wide at the ambitus from which they narrow to the peristome 

 and apex, but with somewhat pronounced constrictions at the ends of the petaloid portions; 

 the anterior paired ambulacra curving posteriorly on approaching the peristomial depression; 

 dorsal portions petaloid, the odd petal being longer than the others, which are equal or nearly 

 equal in length. The poriferous zones are broad, more than half the width of the interporiferous 

 area, moderately apart at their extremities; the pores equal or nearly so, small, elliptical; pairs 

 of pores united by long, narrow, oblique grooves. Beyond the petaloid portions the small 

 narrow plates of those portions give place to broad plates, each of which to and beyond the 

 ambitus has two or three pairs of very small round pores placed obliquely near together at 

 the extreme outer ends of the plates. The interporiferous areas are slightly tumid and stand 

 out in relief . 



The apical system is central or subcentral. The madreporite is large and tumid, central, 

 pentagonal, the posterior edge being straight and coinciding with those of the posterior radial 

 plates. There are four genital pores situated at the edge of the madreporite, the posterior pair 

 being farther apart than the anterior; and there are five small subtriangular radial plates, each 

 perforated by a very small pore. 



The peristome is pentagonal, slightly elongated transversely, opening at the bottom of a very 

 deep, long, flaring transverse depression in the middle of the under surface. The size and shape 

 of this depression varies, but its length at the surface is \isually about one-half the width, and 

 its depth about one-half the height of the test. Its anterior and posterior walls slope at 

 nearly the same angle but in opposite directions toward the peristome forming a broad, deep, 

 V-shaped concavity. 



The periproct is very small and circular, and situated at about midway between the 

 peristome and the posterior margin. 



Numerous small, uniform, imperforate tubercules, surrounded by deep scrobicules, are 

 scattered irregularly over the entire surface of the test. 



Related forms. — Oligopygus wetlierbyi is most closely related to 0. jloridanus from which it 

 can be distinguished by its more depressed form, more petaloid ambulacra, broader poriferous 

 zones, and its deeper, longer and more open peristomial depression. It also resembles 0. halder- 

 mani, but is readily separated by the more anterior position of its periproct. 



From most foreign forms 0. wetlierbyi can be readily separated by its more profound 

 depression around the peristome. 0. costulatus (Desor), its nearest ally, according to De Loriol, 

 has more markedly ridged ambulacra, a more swollen and slightly conical test, and periproct 

 nearer the peristome. 0. ovum-serpentis (Guppy), from supposedly Eocene deposits of Trini- 

 dad, is similar but, as noted by Stefanini, differs in having a less profound peristomial cavity 

 and a more tumid test. 



Localities. — Near Gainesville, Alachua County (De Loriol) ; 6 miles southwest of Gainesville, 

 Santa Fe River, Alachua County; and Johnsons Lime Sink, Levy County, Fla. 



Geologic horizon. — Vicksburg group, lower Ohgocene. 



Collection.— U. S. National Museum (137881a, A; 137881b, B). 



Oligopygus haldermani (Conrad). 



Plate LXXVIII, figures 4a-d, 5a-d. 



Discoidea haldermani Conrad, 1850, Acad. Nat. Sci. Philadelphia Jour., 2d aer., vol. 2, p. 40, PI. I, fig. 12. 

 Discoidea haldermani Conrad, 1S65, Acad. Nat. Sci. Philadelphia Proa, p. 75. 

 Discoidea haldermani Conrad, 1866, Smithsonian Misc. Coll., vol. 7 (200), p. 22. 

 Discoidea haldermani Agassiz, 1883, Mus. Comp. Zoology Mem., vol. 10, No. 1, p. 88. 



Determinative characters. — Test with thick walls, marginal outline oval to subpentagonal 

 and slightly undulating; sides tumid; upper surface usually elevated anteriorly and depressed 



