GLEN DEAN CONODONTS 



17 



tends only part way to the posterior, leav- 

 ing the posterior fourth of the aboral mar- 

 gin sharp-edged. At the posterior end of 

 the oral trough a median carina is typical 

 in C. cristata but is present only rarely in 

 C. convexa and then consists of only a few 

 isolated nodes. The parapets of C. con- 

 vexa are ornamented by sharper more reg- 

 ularly spaced transverse ridges than those 

 of C. cristata. 



There is a considerable range in the size 

 of even the mature specimens of C. cris- 

 tata and in the older specimens the trans- 

 verse ridges of the parapets tend to become 

 less sharp and in some cases remain only as 

 a series of nodes. For these reasons, Elias' 

 variety C. cristata var. grandis is not con- 

 sidered valid. 



Material studied. — 71 specimens. 

 Distribution. — Glen Dean Formation of 

 Illinois, Indiana, and Kentucky, Barnett 

 Formation of Texas, and the Delaware 

 Creek Member of the Caney Shale of Okla- 

 homa. 



Cavusgnathus unicornis Youngquist and 

 Miller 



Plate 1, figures 6-11 



Cavusgnathus unicornis Youngquist and Miller, 

 1949, Jour. Paleontology, v. 23, no. 6, p. 619, 

 pi. 101, figs. 18-23. Rexroad, 1957, Illinois 

 Geol. Survey Rept. Inv. 199, p. 17, pi. 1, fig. 7. 



Cavusgnathus cristata Branson and Mehl, Cooper 

 (part), 1947, Jour. Paleontology, v. 21, no. 2, 

 p. 91, figs. 7-10. 



This easily recognizable species is an im- 

 portant element of the Glen Dean Forma- 



tion at all locations studied. Except for 

 changes during ontogeny, the species shows 

 Httle variation. The young specimens tend 

 to be narrow with parapets nearly straight 

 or slightly concave toward the inner side. 

 With growth the platform broadens with- 

 out a proportional increase in length, there 

 develops a pronounced convexity of the in- 

 ner parapet near the posterior as viewed 

 orally, an increase in the flare of the inner 

 lip of the navel, and an increase in the num- 

 ber of small denticles on the blade anterior 

 to the prominent posterior denticle. 



Material studied. — 524 specimens from 

 this study, 170 from Rexroad 1957 study. 



Distribution. — Renault, Paint Creek, 

 Golconda, Glen Dean, Menard, Clore, and 

 Kinkaid Formations of Illinois, Indiana, 

 and Kentucky, and the Pella Beds of Iowa. 



Repository. — Illinois State Geological 

 Survey, 3P7, 3P8, 3P9, 3P10, 3P11 (figured 

 specimens). 



Genus Gnathodus Pander, 1856 



Type species: Gnathodus mosquensis Pander 



Gnathodus modocensis Rexroad 



Illinois 

 1, figs. 



Plate 1, figures 1 and 2 



Gnathodus modocensis Rexroad, 1957, 

 Geol. Survey Rept. Inv. 199, p. 30, pi. 

 15-17. 



Most specimens of Gnathodus that occur 

 in the Glen Dean Formation are very uni- 

 form in appearance and may be readily 

 identified as G. modocensis. As stated pre- 

 viously (Rexroad, 1957, p. 30), this species 

 closely resembles G. bilineatus (Roundy). 



EXPLANATION OF PLATE 6 

 All figures X 40 



Numbers in parentheses after explanations refer to locality and sample numbers, for example 



(8-7) refers to locality 8, sample 7. 

 Figure 

 1, 2 Ozarkodina compressa Rexroad; 1, inner lateral view (8-7); 2, outer lateral view 



(1-25). 

 3, 4 Spathognathodus cristula Youngquist and Miller; 3, inner lateral view of a mature 



specimen (2-31); 4, inner lateral view of a young specimen (12-7). 

 5-7 Spathognathodus spiculus Youngquist and Miller; lateral views (10-3, 10-3, and 1-43 



respectively). 



8 Spathognathodus cf. S. commutatus Branson and Mehl; inner lateral view (8-7). 



9 Spathognathodus campbelli Rexroad; inner lateral view (12-14). 



10-16 Lambdagnathus fragilidens Rexroad, n.gen., n.sp.; 10, 11, 13, two oblique views and 



an oral view of holotype (1-41); 12, outer lateral view of a paratype (1-49); 14, aboral 

 view of a paratype (1-60); 15, 16, posterior views of two paratypes (12-composite and 

 12-2). 



