280 PALEONTOLOGY OF ILLINOIS. 



ship between the two specimens. Compared with other forms, perhaps 

 this bears the strongest resemblance to C. euglypheus of the St. Louis 

 Limestone; it is, however, distinguished from that form by the fiuer 

 striation and greater depth of the anterior basal region. 



Position and locality : Carbonaceous shale accompanying coal No. 5, 

 at Girard, 111., and bed No. 30 of the middle Coal Measures, Guthrie Co., 

 Iowa. 



Genus LAMBDODUS, St. J. and W. 



Teeth small, base posteriorly produced, more or less laterally expanded, 

 long-elliptical or sub-circular in outline, broadest behind the cornua, 

 slightly produced in front, gently concave, or sometimes convex below, 

 superior surface convex A single strong, slightly sigmoidally curved, 

 recurved, eccentric cornua arises from the anterior angle of the base, 

 terminating in a sharp apex, compressed in front, broadly rounded 

 behind, with more or less distinct cutting edges and irregular vertical 

 cost as. 



The teeth recognized under the above generic term were at first sup- 

 posed to pertain to some part of the dental economy of Cladodus. The 

 accession of material, mainly through the efforts of Mr. Springer, 

 affording a more complete knowledge of the individual characteristics 

 of the teeth of the two most abundantly represented forms, has resulted 

 in the development of no further intimacy with the above genus, while 

 it has conclusively proved the persistency of the characteristics which 

 distinguish them from Cladodus, and lead to the recognition of their 

 generic distinctness. It is distinguished from Cladodus by the single 

 coronal cornua, the absence of lateral denticles. The basal portion bears 

 some resemblance to Thrinacodus, but the coronal features are entirely 

 dissimilar. The genus is, so far as at present known, restricted to the 

 Lower Carboniferous formations, occurring in considerable abundance 

 in the upper Burlington and Keokuk limestones, and extending upwards 

 in the St. Louis and Chester formation. 



Lambdodtjs costatus, St. J. and W. 



PI. V, Fig. 3 



Teeth of small size. Base laterally oval or sub-circular in outline 

 comparatively thin, inferior surface slightly concave, anterior border 

 moderately produced and slightly thickened. Coronal cusp strong, 

 gradually tapering to the acute apex, slightly deflected laterally and 

 sigmoidally curved backward, slightly compressed laterally, ovate sec- 

 tion, with sharp cutting edges well defined towards the apex, but gener- 



