VEKTEBKATES. 153 



The present species is represented by several examples of the. 

 mandibular and maxillary posterior forms, whose perfect state of 

 preservation permits of detail comparison with allied species. It 

 bears most intimate relations with Deltodus cinctulus of the Warsaw 

 beds, the mandibular teeth being chiefly distinguishable by the pro- 

 portionately greater breadth of the principal coronal ridge, narrower 

 posterior wing, and the more numerous transverse furrows and 

 ridges ; in the case of the maxillary posterior form, it contrasts 

 with that of the above species in the relatively more prominent 

 coronal ridge, narrower alation, abrupt deflection in the course of 

 the inner margin, and the less conspicuous, narrow transverse 

 undulations traversing the coronal region. 



A single imperfect specimen of a minute tooth obtained by one 

 of the writers at St. Louis, is provisionally identified with the narrow 

 median form of the mandible of the present species. Partaking in 

 an intimate degree of the outline and coronal contour of the median 

 teeth characteristic of the genus, as it is now interpreted, its small 

 size suggests relationships with the specific forms above alluded to. 

 It shows the, prominent, rounded coronal ridge, narrow alate postero- 

 lateral border, and the still narrower and shorter articular border 

 of the opposite side, but the mutilated condition of the specimen 

 does not disclose the distinctive specific characters that distinguish 

 it from congeneric forms. 



Geological position and localities: St. Louis formation ; Pella, Iowa; 

 Alton, Illinois ; St. Louis, Missouri. 



DELTODUS INTEEMEDIUS, St. J. and W. 



PI. IX, Fig. 14, 15. 



Teeth attain medium size. Maxillary posterior form sub-triangular 

 in outline, rather strongly arched and inrolled. Antero-lateral border 

 slightly curved sigmoidally, articular edge very narrow, inbeveled to 

 the slightly channeled, shallow basal rim ; postero-lateral border 

 converging toward inrolled outer margin at an angle of 35 with the 

 opposite side, somewhat undulated, the relatively thick enamel fold 

 sharply inbeveled to the channeled, posteriorly expanded basal rim, 

 which is rather strongly produced at the subacute, rounded posterior 

 angle ; inner margin strongly produced inward and slightly backward 

 from the very obtuse angle at the junction of the anterior articular 

 border, suddenly deflected nearly at right angles and broadly arched 



