VERTEBRATES. 179 



the maxillary posterior tooth shows obscure indications of parallel 

 transverse disposition of the punctae, though it is not nearly to the 

 same degree apparent as in the typical species of the genus. 



The present species bears a somewhat striking resemblance to 

 Tomodus convexus, Agass., of the Mountain limestone of Great Britain. 

 As has been already intimated, there would seem to be very inti- 

 mate relationship between Xystrodns and Tomodus, the forms of both, 

 especially the mandibular posterior teeth, showing the same outline 

 and contour of the crown, and chiefly distinguished one from the 

 other by the presence or absence of transversely disposed punctse. 

 In the present species the very obscure indications of the disposi- 

 tion of the punctse in rows parallel with the inner margin of the 

 teeth might be regarded as a transitional feature connecting the 

 more widely differentiated species which are accepted as the typical 

 representatives of one or other of the above named genera. 



Geological position and localities: Upper Burlington limestone, fish- 

 bed; Buffington creek, Louisa county; Burlington and Augusta 

 (Iowa) ; Monmouth, Illinois. 



XYSTKODUS INCONDITUS, St. J. and W. 

 PI. viii, rig. i. 



Teeth very small, or below medium size, somewhat strongly 

 arched from within outwards. Antero-lateral border nearly straight, 

 or presenting the usual slight sigmoidal curvature, vertical, meeting 

 in a sharp angle above, over which the enamel folds in a rather 

 deep belt well defined from the basal portion, which inclines slightly 

 outward to the thin inferior edge; inner margin broadly rounded, 

 with a slight concave curvature in passing the coronal depression, 

 the course slightly oblique from the anterior to the posterior angle; 

 postero-lateral border apparently somewhat rapidly converging towards 

 the outer extremity, showing the usual narrow enamel fold along 

 the upper edge, but oftener mutilated from abrasion. Coronal sur- 

 face occupied by a rather wide, shallow depression, the moderately 

 upraised posterior wing presenting a relatively wide 'transverse 

 diameter, on the opposite side rising into the gently arched or lat- 

 erally convex prominence along the antero-lateral border. ^.Triturat- 

 ing surface in almost every case exhibiting indications of excessive 

 wear, in which even the punctate structure is obscure ; along the 

 inner margin, however, the teeth often preserve the smooth, dense, 

 polished enamel coating, but in the middle portion of the crown, 



