EOCENE ECHIXODER1IATA. 123 



extending two-thirds the way to the margin; poriferous zones much narrower than interporif- 

 erous areas. Peristome small, subpentagonal, slightly excentric anteriorly; ambulacral grooves 

 simple, straight, inconspicuous. Periproct small, subcircular, from one-fourth to one- third the 

 way from margin to peristome. 



Dimensions. — Specimen A: Length 21 millimeters; width 26 millimeters; height 5 milli- 

 nieters. Specimen B: Length 31 millimeters; width 26 millimeters; height 6 millimeters. 



Description. — This little echinoid was one of the first reported from American deposits, 

 having been described and figured by Morton as early as 1833. It is relatively somewhat abun- 

 dant in the Eocene deposits and has been found in South Carolina, Georgia, and Alabama. The 

 test is small, ranging. from three-quarters of an inch to 1J inches in length. In marginal outline 

 it is rather variable, being subpentagonal in most specimens but suboval or subovate in some. 

 The whole form is greatly depressed, the upper surface tumid centrally, the tumid area involving 

 the larger part or all of the ambulacral petals; the margin is rounded and notably thickened all 

 around, though slightly less so at the middle of the posterior end than elsewhere, the submarginal 

 area flat or nearly so, as thick as or more or less thick than the margin. The under surface is 

 decidedly concave, the concavity reaching nearly to the margin. The apex is slightly excentric 

 anteriorly at the summit of the tumid area. 



The ambulacral petals are lanceolate or elongate subelliptical, pointed and practically closed 

 at the distal ends; the anterior pah slightly shorter than the other three, all of them extending 

 about two-thirds the way to the margin. The poriferous zones are very narrow, much narrower 

 than the interporiferous areas, sometimes irregular, the inner ends poorly defined, inner row of 

 pores round, outer row slitlike, pairs of pores conjugated. The whole surface of the test, 

 including the interporiferous areas and the apical region is covered with rather conspicuous 

 small tubercles set in deep scrobicules which are larger on the lower surface. 



The apical system is central or slightly excentric anteriorly at the summit of the tumid area. 

 There are four large genital pores, the anterior pair being set closer together than the posterior 

 pair. The poriferous zones rarely if ever can be seen to come together at the proximal ends, and 

 the perforations of the radial plates, if they exist, can not be discerned. 



The peristome is small, subpentagonal, slightly excentric anteriorly; the ambulacral grooves 

 are never very distinct on even well preserved specimens. They appear to be straight, simple, 

 faint lines from peristome to near the margin, distinguished more by finer tuberculation than 

 by the existence of a distinct groove. 



The periproct is small, about one-third the diameter of the peristome, subcircular, situated 

 from one-fourth to one-third the way from the margin to the peristome. 



Related forms. — Of American forms L. (?) crustuloides is most closely related to L. johnsoni 

 which diff ers chiefly in being higher and having a thicker margin. L. ( ? ) crustuloides is also closely 

 related to L.jloridanum which differs in having a smaller petaloidal area, a more regularly oval 

 marginal outline, a concave ring around the ends of the petals and a flat lower surface. 

 Scutella (?) conradi also resembles the present species but differs in having its margin thin and 

 posteriorly truncated and notched. L. archerensis and L. dalli both differ in having flat lower 

 surfaces and the apical region but little if any higher than their thick margins. 



Of foreign forms the present species is allied to Laganum rejlexum from the Tertiary of 

 Blaye, France, which, however, is thinner posteriorly. Sismondia marginalis (Des Moulins) 

 Desor from the middle Eocene of France also resembles the present form but differs in having 

 petals which are longer and more open at the end. 



Localities. — South Carolina, probably near Charleston; near Bainbridge, Ga. ; Dothan, Ala. 



Geologic horizon. — -Jackson foimation, (probably) upper Eocene; Vicksburg formation, 

 lower Oligocene. 



Collections. — -Boston Society of Natural History (SS02a, A); U. S. National Museum (1(34, 

 744, B). 



