VERTEBKATES. 163 



punctation and undulations and the delicate transverse tracery 

 observed in partially exfoliated surfaces, especially of the wing ex- 

 pansion. A specimen of the ordinary size measures in extreme 

 length along the coronal crest 23 mm; antero-lateral border about 16 

 mm; breadth across inner margin 8 mm. 



The present species is of peculiar interest on account of the 

 facts it affords in relation to specific diferentiation and the inti- 

 mate resemblances that exist between the individuals of allied 

 species. Thus, compared with the Chester form there exists mark- 

 edly close resemblance between the mandibular posterior teeth, the 

 chief distinction in the present form being its, perhaps, less oblique 

 inner margin, the peculiar coronal ornamentation and less regular 

 punctate structure. But in relation to the maxillary posterior teeth 

 of the two forms, a marked difference is apparent, the present form 

 being relatively much more elongate longitudinally and proportion- 

 ately narrower, while the proportionately narrow, plane or slightly 

 convex anterior slope contracts with the wide, gently concave 

 declivity characteristic of the Chester teeth. 



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

 Iowa, Alton and other localities in Madison and Monroe counties, 

 Black's Hill, Hardin Co., 111., St. Louis, Mo. 



DELTODOPSIS ANGUSTUS, (N. and W., sp.) * 



PL XI, Fig. 7-10. 

 Deltodus angustus, NEWBERRY and WOETHEN, 1870, 111. Geol. Surv. IV, p. 368, PI. iii, f. 7. 



Maxillary posterior teeth subcuneate in outline, very moderately 

 arched longitudinally, except at the inrolled extremity. The antero- 

 lateral articular border comprising somewhat more than half the 

 extreme length of the tooth, defined by the thick, rounded enamel 

 fold, which is strongly inbeveled to the shallow, channeled basal 

 rim; inner margin very obliquely produced backward from the ob- 

 tuse anterior angle, where it unites with the antero-lateral border, 

 sharply arched round the base of the coronal ridge, with a slight 

 sigmoidal curvature thence to the posterior angle nearly at right- 

 angles to the anterior portion; postero-lateral border gently curved 

 from a right line, very gradually converging toward the inrolled ex- 

 tremity ; coronal enamel forming a very narrow, inbeveled fold, well 

 defined from the shallow-channeled deep basal border, which is 

 usually inbeveled, probably in perfect specimens nearly vertical, 



