20 



ILLINOIS STATE GEOLOGICAL SURVEY 



Fig. 14. — Diagrammatic reconstruction of a mature 

 representative of Lambdagnathus Rexroad, n. 

 gen., showing outer lateral (A), aboral (B) and 

 outer posterior (C) views, X35. 



also is inclined sharply downward. Typi- 

 cally, it is arched, slightly bowed convexly 

 anteriorly, is deepest near its midlength, 

 and has denticles largest near its midsection 

 and decreasing in size in either direction, 

 the larger denticles being considerably 



larger than the apical denticles. Attach- 

 ment scars may be prominent on the faces 

 of the inner lateral process, less so on the 

 posterior bar and anterior process. Chief 

 variations in the genus are expected to be 

 in the proportions of the lengths of the 

 three processes and their angular relations 

 to each other, plus variations in the dentic- 

 ulation and depths of the processes, partic- 

 ularly the inner lateral one. 



Relationships of this genus to other gen- 

 era are problematical. The general form 

 is most similar to that of Centrognathodus 

 Branson and Mehl, but it differs particu- 

 larly in the angular relations of the three 

 processes, in the presence of a pronounced 

 triangular subapical pit in Lambdagnathus, 

 and in its triangular apical denticle. 



Lambdagnathus fragilidens Rexroad, 



n.sp. 



Plate 6, figures 10-16 



New Genus ? (Part), Rexroad, 1957, Illinois Geol. 

 Survey Rept. Inv. 199, p. 41, pi. 4, fig. 10 only. 



Posterior bar delicate, no thicker than 

 the four posteriorly inclined laterally com- 

 pressed denticles it typically bears. Ante- 

 rior process curves outward at juncture of 

 bars, then continues direction of posterior 

 bar. Anterior process thin, of uniform 

 depth, with about twelve denticles which 

 are slightly smaller toward anterior end of 

 process. Inner lateral process viewed orally 

 appears nearly perpendicular to posterior 

 bar, viewed anteriorly is inclined down- 

 ward approximately 45°. Inner lateral 

 process deepest about one-third distance 

 from juncture of processes, with two largest 

 denticles located here, denticles strongly 

 compressed in plane of process, slightly re- 

 curved, and typically number from nine to 

 eleven. Apical denticle larger than den- 

 ticles of posterior bar, triangular in shape 

 with sharp edge in line with each process, 

 slightly recurved. Well defined triangular 

 pit beneath apical denticle with grooves 

 extending from it along the thin aboral 

 margin of each of the processes. 



Material studied. — 19 specimens. 



Distribution. — Glen Dean Formation of 

 Illinois, Indiana, and Kentucky. 



