74 PALEONTOLOGY OF ILLINOIS. 



Geological position and location: St. Louis limestone ; Alton, 111., 

 St. Louis, Mo. 



PSEPHODUS CDNULATUS, St. J. and W. 



PI. II, Fig. 4. 



Tooth small, quadrato-lunate in outline, lateral borders nearly 

 parallel, posterior margin strongly arched forward, making obtuse 

 angles with the lateral borders, anterior margin correspondingly 

 concave, terminating in the acute, outward-produced lateral angles. 

 Coronal region moderately arched in both directions, with an abrupt 

 declivity bordering the anterior side, the surface presenting a some- 

 what uneven contour, which is also discoverable in the slightly greater 

 prolongation and rounding of one of the outer lateral angles, and 

 the relatively greater depth of the tooth at that side ; the enamel 

 makes a distinct narrow belt sharply inbeveled to the basal portion 

 all round, lateral borders obscurely undulated, posterior edge plicato- 

 denticulate, outer margin occupied by vertical plicae terminating below 

 in deep crenulations. The base is relatively thick ; inferior surface 

 slightly depressed and smooth, lateral borders nearly vertical and 

 faintly channeled, inner margin deeply channeled, together with 

 the coronal belt, and terminating below in a thin rim slightly in 

 advance of the coronal edge, the edges all round coarsely pitted. 

 Coronal surface presents a somewhat irregular, minute punctate 

 structure, showing marked evidences of attrition from use. Breadth 

 across outer margin 9.5 m m. ; length along one of the lateral 

 borders 5mm.; length along median line 8mm. 



The unique specimen above described represents a nearly perfect 

 tooth of a form not before met with. Its affinities are unquestion- 

 ably with Psephodus, with whose numerous dental remains it is 

 associated. The narrow forms of P. crenulatus (N. and W., sp.,) 

 present, in their crenulated margins, striking resemblance with the 

 present tooth, even as to the details enumerated above, though 

 perhaps not so pronounced as observed in the individual described. 

 The same observation holds as regards the coronal contour, which 

 is gradually sloped from the brink of the abrupt outer declivity to 

 the inner margin. It is, therefore, with some doubt its specific dis- 

 tinctness is recognized ; it may prove to belong to a posterior row 

 of teeth of one or other jaw, or possibly interposed near the sym- 

 physis between the flat, narrow, Helodas-like teeth, whose specific 

 identity with the contorted, inrolled median plates of P. crenulatus 



