160 MESOZOIC AND CENOZOIC ECHINODERMATA OP THE UNITED STATES.- 



Subclass IREEGULARIA. 



Order GNATHOSTOMATA. 



Suborder CLYPEASTRINA. 



Family FIBULARIIDiE. 



Genus ECHINOCYAMUS Phelsum. 



ECHINOCYAMUS VAUGHANI Twitchell, 11. sp. 



Plate LXXIV, figures la-d. 



Determinative characters. — Test very small, though larger than related American species, 

 thick walled, almost regularly egg shaped, though elongated longitudinally, about twice as long 

 as broad. Upper surface elevated, both upper and lower surfaces flattened along the longi- 

 tudinal median areas. Apical system anteriorly excentric, with four medium-sized genital 

 pores. Petals relatively short and petaloidal area smaller than in related forms. Peristome 

 relatively large, central, slightly depressed; periproct very small, close to peristome. 



Dimensions. — Length, 8 millimeters; width, 4 millimeters; height, 4 millimeters. 



Description. — This species is founded upon a specimen collected in Georgia by T. Wayland 

 Vaughan, of the United States Geological Survey, in whose honor it is named. The test, 

 though actually very small, is larger than in the other American representatives of the genus. 

 The test is thick walled, elongate egg shaped in general form, elongate subelliptical to elongate 

 subovate in marginal outline, somewhat pointed anteriorly, about twice as long as broad. The 

 upper surface is high (the height of the test equalling the breadth) and flattened longitudinally; 

 under surface also flattened somewhat along the longitudinal median area and slightly concave 

 around the peristome. 



The ambulacral petals are rather well defined, relatively short, thus making the petaloidal 

 area relatively smaller than in allied American forms; petals wide open at ends; poriferous zones 

 diverge in almost straight lines to the ends and consist of small round pores in pairs not dis- 

 tinctly conjugated. 



The peristome is relatively large, central, slightly depressed below the surface; periproct 

 very small, about one-third the diameter of the peristome and very close to the peristome. 



Related forms. — E. vaughani is most closely related to E. texanus but can be readily sepa- 

 rated by its larger size, less oval marginal outline, more pointed anterior end, greater difference 

 between its longitudinal and transverse diameters, and smaller petaloidal area. E. vaughani 

 also resembles E. parvus and E. huxleyanus but differs from both in having its periproct near 

 the peristome. It also differs from E. huxleyanus in being less fragile and less depressed. 



Locality. — Little Horseshoe Bend, Flint River, Ga. 



Geologic horizon. — -Vicksburg formation, lower Ohgocene. 



Collection.— U. S. National Museum (1664S6). 



Family LAGANIDiE. 



Genus LAGANTTM Blainville. 



Laganum eldridgei Twitchell, n. sp. 



Plate LXXIV, figures 2a-d. 



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

 longitudinally elongate, truncated at the anterior and posterior ends, more or less undulating 

 along the sides. Whole form greatly depressed, margin thin but thicker than slightly concave 

 submarginal area, petaloidal region tumid. Apex and apical system subcentral. Lower surface 

 flat. Ambulacral petals long, elongate elliptical, extending two-thirds the way to the margin, 

 pointed and closed at the ends; pairs of pores conjugated by very narrow more or less wavy 

 grooves. Ambulacral areas very wide at margin, narrowing rapidly to ends of petals. Peri- 

 stome moderate in size, subcentral, subpentagonal to subelliptical, transversely elongate. 



