VERTEBRATES. 183 



wavy, coalescing and bifurcating sharp transverse rugse. The linear 

 parallel disposition of the punctae may be detected even in much 

 worn surfaces, though, as might be expected, it is here less apparent ; 

 and toward the outer extremity, in excessively worn teeth, the 

 surface structure does not differ from that usually met with in the 

 Cochliodonts. 



The teeth above attributed to the posterior position on the max- 

 illary are comparable to Xystrodus striatus, Agass., from which they 

 are chiefly recognizable by the proportionately narrower outline and 

 perceptibly less expanded alation. The opposed teeth, as herein 

 interpreted, are also intimately allied to a form associated with X. 

 striatus, of which the collections of Lord Enniskillen at Florence 

 Court, contain numerous representatives. The latter form, with the 

 original X. striatus, presents exactly the same association of forms 

 which we have attempted to ascribe to the more prominent dental 

 elements of this genus, as represented in the present species. The 

 Chester tooth, however, differs from the extremely alate examples 

 from Armagh in its great length as compared with the inner trans- 

 verse diameter, or in directly opposite character to the distinguishing 

 features noted in connection with the maxillary teeth. 



Geological position and localities : Chester limestone ; Chester, and 

 Monroe county, 111. 



XYSTRODUS BELLULUS, St. J. and W. 



PI. VIII, Fig. 3. 



The form here indicated is represented by a small tooth, some- 

 what imperfect at the apical extremity and along the oblique border 

 and the inner margin, pertaining to the left ramus of the maxillary. 

 Very gently arched longitudinally, narrow wedge-shaped in out- 

 line and very gradually tapering, crown broadly arched laterally, an 

 obscure angulation in the median depression defining the anterior 

 coronal prominence from the narrow, slightly upraised alate expan- 

 sion along the oblique border; along the straight or antero-lateral 

 border the crown is rounded and then sharply inbeveled from the 

 distinct angulation, the narrow enamel fold limited by a slight fur- 

 row, beneath which the basal rim is gently produced outward and 

 downward, terminating in a thin, rounded edge. The inferior sur- 

 face of the tooth is smooth, approximately conforming in contour 

 to that of the crown. The coronal surface is beautifully marked by 



