170 MESOZOIC AND CENOZOIC ECHINODERMATA OF THE UNITED STATES. 



Numerous small, uniform, imperforate tubercles, surrounded by deep scrobicules, are 

 scattered irregularly over the entire surface of the test. 



Related forms. — OUgopygus fl.oridanus is closely related to 0. weiherbyi, from which it can be 

 separated by its higher and less flattened upper surface, its subpetaloid ambulacra, narrower 

 poriferous zones, and its shorter, less flaring, anteriorly projecting peristomial depression. It 

 also resembles OUgopygus TiaVlermani but can at once be distinguished by the more anterior 

 position of its periproct. 



Of foreign forms, 0. ovum-serpentis (Guppy), from supposedly Eocene deposits of Trinidad, 

 is closely allied; but its periproct is nearer the margin and its peristomial depression less 

 pronounced. 



Locality. — Nigger Sink, 2 miles south of Gainesville, Alachua County, Fla. 



Geologic horizon. — Vicksburg group, lower Oligocene. 



Collection.— U. S. National Museum (164660). 



Family CASSIDULIDJE. 



Genus CASSIDULTJS Lamarck. 



Cassiduxus (Pygorhynchtts) georgiensis Twitchell, n. sp. 

 Plate LXXIX, figures 2a-d. 



Determinative characters. — Test broadly oval, upper surface convex, moderately elevated, 

 sides and anterior end rounded; posterior end obliquely truncated, under surface flat, curving 

 upward slightiy posteriorly to meet the posterior truncation in an angular margin; apex sub- 

 central. Ambulacral areas narrow; petals narrow, nearly closing, the posterior pair having the 

 inner poriferous zone of each petal shorter than the outer zone. Apical system excentric ante- 

 riorly. Peristome excentric anteriorly, pentagonal, transversely elongate, with a floscelle. 

 Periproct small, subrhomboidal, transverse; located at the top of the rather high posterior trun- 

 cation, beneath a rounded, somewhat protruding expansion of the test. 



Dimensions. — Length 26 millimeters; width 22.5 millimeters; height 15 millimeters. An- 

 other specimen, of nearer average size gave, length 35 millimeters; width 32 millimeters; height 

 20 millimeters. 



Description. — This new Cassidulus, which is rather rare, only three specimens being reported, 

 has a test which is broadly oval in marginal outline, more obtusely rounded posteriorly than 

 anteriorly, and obliquely truncated at the posterior end. The upper surface is regularly convex, 

 moderately elevated, in the form of a low, rounded ridge above the periproct, sides and anterior 

 end rounded and inflated; under surface flat, curving upward slightly posteriorly to meet the 

 oblique posterior truncation in an angular margin, the angle formed being about 75°. The apex 

 is subcentral. 



The ambulacral areas are narrow, dorsal portions petaloid, the petals narrow, partly open 

 at the ends, the posterior petals longer than the others, which are nearly equal in length. The 

 poriferous zones are narrow; the inner zone of each of the posterior pair of petals shorter than the 

 outer zone; outer row of pores slitlike, inner row round; pairs of pores conjugate. 



The surface of the test is closely set with numerous small tubercles which increase in size 

 on the under surface except along a rather wide median band which is free from tubercles and 

 dotted with numerous small pits. The tubercles are set in scrobicules which are shallow and 

 irregularly shaped on the upper surface; but become larger, deeper and more regular in form on 

 the under surface. 



The apical system is excentric anteriorly. The only details which can be made out are the 

 existence of four genital pores, of which the anterior pair are nearer together than the posterior, 

 and five small perforated radial plates. 



The peristome is excentric anteriorly, immediately beneath the apical system, pentagonal, 

 transversely elongate, with a floscelle of which the phyllodes are rather well defined and the boui- 

 relets are large and prominent. 



