18 



ILLINOIS STATE GEOLOGICAL SURVEY 



This is especially apparent when compar- 

 ing the adult of the former with the young 

 of the latter. This resemblance seems to 

 indicate that the two are closely related al- 

 though not conspecific because of the differ- 

 ences between adults. About five percent 

 of the specimens of Gnathodus found in 

 the Glen Dean bear an especially strong 

 resemblance to G. hilineatus (Roundy). 

 These specimens, nevertheless, are identi- 

 fied as G. modocensis and are considered 

 variants toward the closely related species. 



Material studied. — 87 specimens. 



Distribution. — Paint Creek and Glen 

 Dean Formations of Illinois, Indiana, and 

 Kentucky. 



Repository. — Illinois State Geological 

 Survey, 3P12, 3P13 (figured specimens). 



Gnathodus ? sp. 

 Plate 1, figures 3-5 



Four specimens from two localities are 

 difficult to place generically because they 

 are young and broken. From oral view a 

 free blade continues as a slightly sinuous 

 carina with nearly symmetrical parapet-like 

 units on each side. On the aboral side of the 

 platform is a navel which in the broken 

 specimens gives the impression of being 

 asymmetrical, the inner side being essen- 

 tially limited to the under side of the inner 

 parapet and the outer side perhaps extend- 

 ing outward as a platform beyond the outer 

 parapet-like structure on its oral surface. 

 This development is similar to that in sev- 

 eral species of Gnathodus including G. 

 girtyi Hass from the Barnett Formation of 

 Texas. 



Material studied. — 4 specimens. 



Distribution. — Glen Dean Formation of 

 Illinois, Indiana, and Kentucky. 



Repository. — Illinois State Geological 

 Survey, 3P14, 3P15, 3P16 (figured speci- 

 mens). 



Genus Hibbardella Bassler, 1925 



Type species: Hibbardella angulata Hinde 



Hibbardella milleri Rexroad, n.sp. 



Plate 2, figures 13-16 



Hibbardella n.sp. ? Rexroad, 1957, Illinois Geol. 



Survey Rept. Inv. 199, p. 31, pi. 1, fig. 19. 



Anterior arch moderately deep, charac- 



teristically with a small central denticle im- 

 mediately anterior to main cusp, each lat- 

 eral bar typically bearing four discrete 

 round denticles bowed posteriorly and in- 

 creasing in size distally, distal ends of bars 

 spatulate; aboral edges of lateral bars thin, 

 grooved, may be slightly sigmoidal as 

 viewed from anterior and meeting at base 

 of cusp in an acute angle, bars inclined 5° 

 to 20° posteriorly; anterior faces convex 

 and inclined posteriorly downward. Cusp 

 large, recurved, elliptical in cross section, 

 sharp edged fore and aft, but increasingly 

 rounded toward tip. Posterior bar short, 

 anterior end of bar stout in adults, longer 

 and thinner in young, few denticles or 

 nodes on oral surface in adults, marked 

 attachment scars on anterior end of bar 

 seen in lateral view; aboral margin wedge- 

 shaped with longitudinal median groove. 



The species is named in honor of A. K. 

 Miller of the State University of Iowa. 



Material studied. — 113 specimens from 

 Glen Dean, 25 from Rexroad 1957 study. 



Distribution. — Renault, Paint Creek, 

 Golconda, Glen Dean, Menard, Clore, and 

 Kinkaid Formations of Illinois, Indiana, 

 and Kentucky. 



Repository. — Illinois State Geological 

 Survey, 3P17 (holotype), 3P18, 3P19, 3P20 

 (para types). 



Hibbardella ortha Rexroad, n.sp. 

 Plate 2, figures 9-12 



Anterior arch deep, anterior face coin- 

 ciding with anterior face of main cusp; 

 the two lateral limbs nearly in a common 

 plane, anterior faces flat, each limb typi- 

 cally with five erect primary denticles fused 

 about one-half way to apices, some speci- 

 mens with germ denticles between; aboral 

 edges of lateral limbs thin, meeting at base 

 of cusp at a low obtuse angle. Cusp erect, 

 thin, nearly straight, triangular in cross sec- 

 tion with flat anterior face and a sharp edge 

 posterior. Posterior bar moderately deep, 

 thin, none complete, bears closely spaced, 

 laterally compressed denticles; aboral mar- 

 gin with median longitudinal groove which 

 extends anteriorly to tiny aboral pit near 

 the anterior of base of cusp. 



