VERTEBRATES. 87 



to the posterior angle; postero-lateral border, basal portion chan- 

 neled and expanded behind. The principal lobe occupies something 

 less than half the transverse diameter of the crown, presenting a 

 nearly plane or slightly convex slope to the abrupt anterior edge, 

 along which should lie the distinctly defined narrow, plain belt did 

 these teeth belong to Deltoptyckius, and where, indeed, in some 

 specimens a faint depressed line does occur, a character in accord- 

 ance with the homologous form of the latter genus; posteriorly the 

 slope more abruptly descends into the wide posterior depression, 

 along the outer border of which lies the rather suddenly upraised 

 posterior wing. The surface presents a similar punctate appearance 

 noted in connection with the before mentioned maxillary teeth. 

 Breadth across inner margin, 5mm; length of antero-lateral bor- 

 der to point of inrollment, about 5 mm. 



The maxillary teeth above noticed present an approach to the 

 straight postero-lateral border characteristic of OrtJwpleurodus ; but 

 here the resemblance ceases, for the border is gently arched, and 

 the position of the posterior lobe, or rather the anterior culmina- 

 tion of its crest, determines its relations with the present genus, 

 should it prove not to be referable to Deltoptyckius. Their exist- 

 ence, however, is extremely suggestive of the derivation of the coal- 

 measure genus, although there seems to be evidence that the present 

 genus survived the introduction of Ortkopleurodus, along with which 

 its supposed representatives are found. 



Compared with the previously- described forms from the St. Louis 

 limestone, V. simplex, there exists the most intimate relationship, if 

 not actual specific identity, the chief distinction consisting in the 

 apparent greater robustness of the present teeth. 



With the above observations, until more complete materials shall 

 have been acquired, the determination of the generic identity of 

 these fragmentary dental remains will necessarily remain for the 

 present in a state of uncertainty. So intimately are the species of 

 Vaticinodus, Stenopterodus and Deltoptyckius linked together, that it 

 might be anticipated the more differentiated forms of Ortkopleurodus 

 may have had ancestors in direct line as early as the time when 

 the sediments of the St. Louis and Chester formations were in 

 process of deposition. 



Geological position and locality : Chester limestone, upper fish-bed 

 stratum; Chester and Evansville, Illinois. 



