EOCENE ECHINODERMATA. 161 



Anibulacral grooves apparently simple and straight, each groove having a fine line on both 

 sides which rapidly diverge from the main groove. Periproct relatively large, suboval, longi- 

 tudinally elongate, one-fourth the wa}^ from the margin to the peristome. 



Dimensions.— Length 96 millimeters; width 75 millimeters; height 10 millimeters. 



Description.— The only specimens known of this large and handsome Laganum, the largest 

 representative of the genus from American deposits, were collected in Florida by the late 

 G. II. Eldridge of the United States Geological Survey and the species is therefore named in 

 his honor. The test is large, being 3 to 4 inches long and 2i to 3 inches wide. In marginal 

 outline it is subpentagonal to decagonal, decidedly longer than broad; margin thin but thicker 

 than the submarginal area, truncated at anterior and posterior ends, more or less undulating 

 along the sides. The whole form is greatly depressed, the upper surface slightly concave around 

 the submarginal area, arising centrally in the form of a low mound which involves the whole 

 petaloidal area; apex subcentral; lower surface flat. 



The ambulacral areas are very wide at the margin, being there about four times the width 

 of the interambulacral areas, narrowing rapidly to near the ends of the petals. The petals are 

 long, elliptical in outline, extending two-thirds the way to the margin, pointed and closed at 

 the ends. The anterior pair shorter than the other three, which are subequal in length. The 

 poriferous zones are wide, more than half the width of the rather narrow interporiferous areas, 

 inner row of pores elliptical, outer row very narrowly slitlike, hardly discernible, pairs of pores 

 conjugated by very narrow more or less wavy grooves. 



The interambulacral areas are very narrow at the margin, widen rather rapidly to near 

 the ends of the petals, and narrow again toward the apical region. The whole test is closely 

 set with very small tubercles, among which are scattered at irregular distances some larger ones 

 in deep scrobicules. 



The apical system is subcentral, at the summit of the tumid area. There are four large 

 genital pores, of which the anterior pair are set closer together than the posterior Dair. No 

 other details could be made out on the specimens. 



The peristome is of moderate size, subcentral, slightly to the rear of the apical system, 

 subpentagonal to subelliptical, transversely elongate. The details of the ambulacral grooves 

 could not be made out satisfactorily on the specimens studied. As they leave the peristome 

 there appears to be a single straight groove on each side of which are two fine lines which rapidly 

 diverge from the main groove. These side lines may be the boundary lines of what on better 

 preserved specimens would prove to be finely granulated areas, thus forming a peristomial star. 



The periproct is relatively large, abdut half the size of the peristome, suboval, longitudinally 

 elongate, situated about one-fourth the way from the margin to the peristome. 



Related forms. — L. eldridgei is readily distinguished from all other American foi - ms. It 



closely resembles L. elongatum Agassiz, 1 whose origin is unknown and which is not even stated 



to be fossil, being very similar in size, general form, and marginal outline, but differs from it 



in having a suboval instead of circular periproct, placed a little farther from the posterior margin. 



Locality. — Suwanee River, Fla. i ■ > - 



. . . 



Geologic horizon. — Vicksburg group, lower Oligocenc. 



Collection.— U '. S. National Museum (164683). 



Laganum aecherensis Twitchell, n. sp. 

 Plate LXXV, figures la-d. 

 Determinative characters. — Test small, subpentagonal to subdecagonal in marginal outline. 

 General form subdiscoidal, upper surface very low, nearly parallel with lower surface, with a 

 slightly concave ring involving most of ambulacral petals between the faintly tumid apical 

 region and the notably thickened margin. Highest part of test along anterior margin. Lower 

 surface flat. Ambulacral petals subelliptical, extending two-thirds the way to the margin, 

 ends closed. Apical system slightly excentric anteriorly, with four genital pores set unusually 

 far apart. Peristome small, pentagonal, central; ambulacral grooves simple, straight lines 



1 Monographies dVchinodermes vivans et fossiles, vol. 2, Des scutelles, pp. 117-118, 1 1. XXIV, figs. 1-2. 



