164 MESOZOIC AND CENOZOIC ECHINODERMATA OF THE UNITED STATES. 



petaloidal area. It differs further from L. crustuloides in having a flat under surface, a less 

 elevated central area, and a concave ring on the upper surface. 



Locality. — Johnsons Sink, Fla. 



Geologic horizon. — Vicksburg group, lower Oligocene. 



Collection.— U. S. National Museum (137884). 



Laganum dalli Twitchell, n. sp. 

 Plate LXXV, figures 4a-d. 



Determinative characters. — Test small, subpentagonal in marginal outline. General form 

 subdiseoidal, the upper surface very low, almost parallel with the flat lower surface. Apex 

 and apical system slightly excentric anteriorly. Ambulacral petals broad, subovate, extending 

 two-thirds the way to the margin; poriferous zones wide, depressed below the general surface, 

 thus making the narrow interporiferous areas stand out in relief; madreporite tumid. Peri- 

 stome probably subcentral. Periproct small, circular, about midway between margin and 

 peristome. 



Dimensions. — Length 16 millimeters; width 15 millimeters; height 4.5 millimeters. 



Description. — The type of this handsome little Laganum was found at Archer, Fla., by 

 W. H. Dall of the United States National Museum, in whose honor the species is named. This 

 species occurs in association with Laganum archerensis. The test is small in size and subpen- 

 tagonal in marginal outline. The whole form is greatly depressed, subdiseoidal, the upper 

 surface being almost parallel with the lower. The margin is notably thickened, slightly less so 

 at the middle of the posterior end. The lower surface is flat or nearly so . The apex is subcentral 

 or very slightly excentric anteriorly, though it is but slightly higher than the margin of the test. 

 There is no distinct concave ring on the upper surface, but the poriferous zones of the ambu- 

 lacral petals are slightly depressed below the general surface. 



The ambulacral petals are subovate in outline, broad, extending nearly two-thirds the way to 

 the margin, rounded, blunt, and closed at the ends. The poriferous zones are very wide, as 

 wide as the interporiferous areas, slightly depressed below the general surface; inner row of 

 pores round, outer row slitlike, pairs of pores conjugated by very narrow grooves. Inter- 

 poriferous areas narrow, standing in relief by reason of the depression of the poriferous zones. 

 The whole surface of the test, including the interporiferous areas, is covered with small tubercles, 

 set in deep scrobicules, which are larger on the under surface. 



The apical system is subcentral or slightly excentric anteriorly, coincident with the apex. 

 There are four large genital pores of which the anterior pair are nearer together than the 

 posterior pair. The madreporite is tumid. No other details can be made out. 



The under surface is damaged on the single specimen known and the details in regard to 

 the peristome and ambulacral grooves can not be given, though it is evident that the peristome 

 was central or subcentral in position. 



The periproct is small, circular, about midway between the margin and the peristome. 



Related forms. — L. dalli is most closely related to L. g,rcherensis, differing chiefly in having 

 no concave ring on the upper surface and in having its periproct farther from its margin. 



L. dalli closely resembles Sismondia antillarum Cotteau from the Eocene of the island of 

 St. Barthelemy but is distinguished by being thicker as a whole, especially on the borders. 



Locality. — Archer, Fla. 



Geologic horizon. — Vicksburg group, lower Oligocene. 



Collection— -TJ '. S. National Museum (164667b). 



