t OLIGOCENE ECHINODEEMATA. 163 



lanceolate, the anterior three subelliptical, all of them pointed and closed at the ends, extending 

 two-thirds or more of the way to the margin, the anterior pair slightly shorter than the rest. 

 The poriferous zones are very narrow, much narrower than the slightly tumid interporiferous 

 areas, sometimes irregular, inner ends poorly defined, inner row of pores round, outer row 

 slitlike, pairs of pores conjugated. The whole surface of the test is covered with rather con- 

 spicuous small tubercles which are larger on the lower surface. The apical system is sub- 

 central, with four large genital pores. The peristome is small, subpentagonal, subcentral; 

 ambulacral grooves poorly denned, not well preserved on the specimen. The periproct is 

 small, subcircular to subpentagonal, about one-third the way from the margin to the peristome. 



Dimensions. — Length 34 millimeters; width 30 millimeters; height 10 millimeters. 



Related forms. — This species is very closely related to L. crustuloides, in fact so close that 

 further collecting may lead to its being made a synonym of the latter species. Its chief dis- 

 tinguishing features are its greater height and the greater height and thickness of its margin. 

 These features distinguish it not only from L. crustuloides, but also from the other American 

 representatives of the genus, from which it differs also in its pronouncedly concave lower surface. 



Locality. — Turks Cave, Ala. 



Geologic horizon. — Probably Vicksburg formation, lower Oligocene. 



Collection. — U. S. National Museum (137202); collected by L. C. Johnson, after whom it is 

 named. 



Laganum floridanum Twitchell, n. sp. 

 Plate LXXV, figures 3a-d. 



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

 the upper surface very low, almost parallel with the lower surface, with a slightly concave ring 

 between the slightly tumid petaloidal area and the notably thickened margin. Apex and 

 apical system slightly excentric anteriorly. Lower surface flat. Ambulacral petals subelliptical, 

 very short, extending halfway to the margin, pointed and closed at ends. Petaloidal area notably 

 smaller than in related forms. Periproct small, circular, about one-third the way from margin 

 to peristome. 



Dimensions. — Length 20 millimeters; width IS millimeters; height 3 millimeters. 



Description. — This new Laganum from Florida has a small test which is almost regularly 

 oval in marginal outline. The whole form is greatly depressed; subdiscoidal; the upper sur- 

 face being almost parallel with the lower; the apical region slightly tumid, the tumidity involv- 

 ing the larger part of the petals; the region around the ends of the petals concave; the margin 

 notably thickened, slightly more so anteriorly than posteriorly. The lower surface is flat, or 

 nearly so. The apex is slightly excentric anteriorly, at the summit of the central tumid area, 

 which rises but very slightly above the height of the margin. 



The ambulacral petals are subelliptical, somewhat pointed and closed at the ends; very 

 short, extending only about halfway to the margin, subequal in length. The poriferous zones are 

 very narrow, much narrower than the interporiferous areas, the proximal ends poorly denned, 

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

 the test is covered with small tubercles set in deep scrobicules, which are somewhat larger on 

 the lower surface. 



The apical system is slightly excentric anteriorly, coincident with the apex. There are 

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

 The poriferous zones can not be seen to come together at their proximal ends, and the perfora- 

 tions of the radial plates can not be made out. 



The peristome is small, slightly excentric anteriorly, subpentagonal. The ambulacral 

 grooves are inconspicuous and could not bo made out on the single specimen studied. 



The periproct is small, about half the diameter of the peristome, circular, located about 

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



Related forms. — L. floridanum is closely related to L. archerensis and to L. crustuloides, but 

 differs from both in being more regularly oval in marginal outline and in having a smaller 



