162 MESOZOIC AND CENOZOIC ECHINODEKMATA OF THE UNITED STATES. 



flanked by finely granulated areas, the five constituting a peristomial star. Periproct small, 

 transversely suboval, about one-fourtli the way from margin to peristome. 



Dimensions. — Length 18 millimeters; width 16 millimeters; height 3.5 millimeters. 



Description. — Only a single specimen of this new Laganum has been discovered. It was 

 found at Archer, Fla., whence the name here given. The test is small, subpentagonal or sub- 

 decagonal in marginal outline. The whole form is greatly depressed; subdiscoidal, the upper 

 surface being practically parallel with the lower; the apical region faintly tumid, the area 

 between the apical region and the margin slightly concave; the margin notably thickened, 

 slightly less so posteriorly than anteriorly. The lower surface is flat. The highest part of the 

 test is along the surface of the anterior margin, from whence the upper surface slopes gently 

 to the posterior margin, the central tumidity being hardly any higher than the side margins. 



The ambulacra! petals are subelliptical, extending about two-thirds the way to the margin, 

 the larger part of each petal being depressed so as to form a part of the concave ring on the 

 upper surface, the ends of the petals practically closed. The poriferous zones are rather wide, 

 about hah as wide as the interporiferous areas, the proximal ends poorly defined, the inner 

 row of pores round, the outer row slitlike, the pairs of pores conjugated. 



The interambulacral areas are very narrow, especially at the margin, where they are hardly 

 more than one-third as wide as the ambulacral areas. The whole surface of the test, including 

 the interporiferous areas and apical region, is covered with small tubercles set in deep scrobicules, 

 which are larger on the under surface. 



The apical system is slightly excentric anteriorly at the summit of the faintly tumid central 

 area. There are four large genital pores, set unusually far apart, the anterior pair being nearer 

 together than the posterior pair. The poriferous zones could not be seen to come together at 

 the proximal ends, and the perforations in the radial plates could not be discerned on the 

 specimen. 



The peristome is small, central, pentagonal. The ambulacral grooves each consist of a 

 central impressed line and two narrow areas of fine granulation which are bounded by faint 

 lines; these grooves are quite distinct for a short distance from the peristome, thus constituting 

 a well-defined peristomial star, they then become gradually less distinct, so that as the margin is 

 approached they can hardly be discerned. 



The periproct is small, about half the diameter of the peristome, transversely suboval, 

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



Related forms. — L. archerensis is most closely related to L. dalli, but can be easily dis- 

 tinguished by the concave ring on its upper surface, by its narrower petals, its less depressed 

 and narrower poriferous zones, and by having its periproct closer to the margin. It is also 

 allied to L. Jloridanum, but has a less regularly oval marginal outline and a larger petaloidal 

 area. It differs from L. crustuloides hi being lower centrally, in having a concave ring on its 

 upper surface, and in having a flat lower surface. 



Locality. — Archer, Fla. 



Geologic horizon. — Vicksburg group, lower Oligocene. 



Collection.— U. S. National Museum (164667a). 



Laganum johnsoni Twitckell, n. sp. 



Plate LXXV, figures 2a-d. 



Description and determinative characters. — The test of this new Laganum is moderate in 

 size and almost regularly oval in marginal outline. The upper surface is moderately elevated 

 centrally, the height being about one-third of the width, the tumid area extending to the ends 

 of the petals; the submarginal area about equal in thickness to the margin, which is slightly 

 undulating, very thick, high and rounded, thicker and higher than in related forms, slightly 

 thinner at the middle of the posterior end than elsewhere. The lower surface is decidedly 

 concave, the concavity reaching nearly to the margin and near the peristome being about 

 equal to one-half the height of the test. The apex is subcentral. The posterior petals are 



