190 PALAEONTOLOGY OF ILLINOIS. 



other in outline and coronal contour exactly after the manner 

 noticed in the Keokuk and Burlington species. The tooth subse- 

 quently described under the designation Sandalodus crassus, N. and 

 W., from the same formation, is identical with the former described 

 form, the original account of S. spatulatiis being founded upon an imper- 

 fect, excessively worn tooth. The collection of Mr. Van Home contains 

 a splendid suite of the latter forms, in all stages of growth and con- 

 ditions of preservation, permitting of the most satisfactory study of 

 their normal peculiarities and variations, so far as this may be 

 made from detached and scattered teeth. 



Geological position and localities: St. Louis limestone ; Alton, and 

 Monroe county, 111., St. Louis, Mo. 



GENUS ORTHOPLEURODUS, 1 St. J. and W. 



Posterior teeth of the upper jaw subspatulate in general outline; 

 postero-lateral border straight or nearly so, and probably gently 

 curved downward and inward at the outer extremity and gently 

 arched in the same direction, terminating posteriorly in an acute 

 angle or spur, whence the inner margin, which is greatly thickened 

 or massive, is broadly rounded into and merging with the thin 

 antero-lateral border towards the extremity. Coronal surface occu- 

 pied by a prominent principal fold or ridge rising nearest the 

 straight border, and flanked on the anterior slope by an obscure 

 secondary ridge; the punctate enamel forms a distinct narrow fold 

 along the thickened straight border, where it is distinctly defined 

 from the basal portion by a slight groove. 



Teeth supposed to have occupied a similar position on the man- 

 dibles, distinguished by their trigonal outline, somewhat strong and 

 spiral inrollment of the extremity, toward which the antero and 

 postero-lateral borders regularly converge, inner margin more or less 

 obliquely rounded and sigmoidally curved from front towards the 

 posterior angle. Coronal surface presenting a more or less w r ell- de- 

 fined plane anterior fold, abruptly broken down on that side, where 

 the coronal enamel forms a wide belt sharply denned from the deep 

 basal rim, and limited behind by the more or less deep longitudinal 

 depression from which rises the alate posterior lobe, which is limited 

 exteriorly by a narrow fold of enamel separating the crown from the 

 basal portion of the tooth. 



KOrthos, straight; pleuron, side; orloux, tooth; in allusion to the straight postero-late- 

 ral border of the maxillary posterior tooth.) 



