EOCENE ECHINODEKMATA. 139 



Genus ECHINANTHUS Leske. 



ECHINANTHUS GEORGIENSIS Twitchell, n. Sp. 



Plate LXV, figure la-d. 



Determinative characters. — Test subpentagonal, broadest posteriorly, anterior end straight, 

 posterior end pointed, notched and rostrate, upper surface depressed, convex, with a longi- 

 tudinal keel along the posteromedian area; under surface concave around the peristome; apex 

 forward of the center. Ambulacral areas narrow; dorsal portions petaloid, petals narrow and 

 rather short. Apical system small, anteriorly excentric. Peristome subpentagonal, transversely 

 elongate, anteriorly excentric, with well-developed floscelle. Periproct small, oval, marginal, at 

 the top of a vertical sulcus which notches the posterior end, and beneath a slightly overhanging 

 projection of the keel. 



Dimensions. — Length 42 millimeters; width 41 millimeters; height 18 millimeters. 



Description. — The species, the first of the genus Echinanthus to be reported from the United 

 States, is very rare, only a single specimen having been found up to the present time. It is of 

 especial interest because it is one of the earliest of the Cenozoic echinoids, the type having been 

 found in the Midway formation of Georgia. The test is subpentagonal in marginal outline, 

 broadest back of the center, from which it narrows both anteriorly to the straight anterior edge 

 and posteriorly to the pointed posterior end; margin angular all around, rostrate posteriorly, 

 with a notch at the posterior end which can be seen from both above and below. The upper 

 surface is depressed, convex, declining rather steeply on all sides from the apex, with a rounded 

 longitudinal keel along the posteromedian area, on both sides of which the surface is obliquely 

 flattened; under surface concave around the peristome. The apex is slightly forward of the 

 center. 



The ambulacral areas are narrow throughout their whole length; dorsal portions petaloid; 

 petals narrow, rather short, partly closing, the posterior pair slightly longer than the anterior 

 pair which are longer than the odd petal. The poriferous zones are narrow, outer pores oval, 

 inner ones round, pairs of pores conjugate. 



The apical system is small, excentric anteriorly, slightly more so than the apex. There are 

 four genital pores, of which the anterior pair are nearer together than the posterior; and there 

 appear to be five very small perforated radial plates. The madreporite is very small and flush 

 with the surface. 



The surface of the test is closely set with very small, apparently imperforate tubercles with 

 deep scrobicules. The tubercles are slightly larger on the under surface except along a rather 

 narrow nearly bare median band back of the peristome. 



The peristome is excentric anteriorly, beneath the apical system, subpentagonal, trans- 

 versely elongate, with a well-developed floscelle. 



The periproct is small, oval, the greater diameter being vertical; marginal, or slightly supra- 

 marginal, beneath a slightly overhanging projection of the keel, at the top of a vertical sulcus 

 which notches the posterior end of the test. 



Related forms. — Echinanthus georgiensis is quite distinct from all other American echinoids, 

 the notched posterior end and the details of the periproct affording a ready means of separation 

 from all forms which in any way resemble it. Among foreign forms it resembles Echinanthus 

 subcarinatus Goldfuss from the Oligocene of Germany but has a less regularly oval outline and a 

 less pronounced posterior rostration. 



Locality. — Five miles south of Ellaville, Ga. 



Geologic horizon. — Midway formation, lower Eocene. 



Collection. — U. S. National Museum (1656836b). 



