VERTEBRATES. 177 



The genus as herein defined offers intimate relationship with the 

 genera Deltoptychius and Orthopleurodus, especially in the conforma- 

 tion and the distribution of the coronal enamel of the mandibular 

 posterior teeth ; but in other and essential characteristics it presents 

 unmistakable features removing its forms from any of the groups of 

 teeth of contemporaneous genera with which they are associated. 

 At the same time we are cognizant of the diversity of the dental 

 armature of the jaws of the Cochliodonts, and the ''missing links" 

 which are required in most instances before the restoration of the 

 dentition of these genera can be consummated. But the teeth at 

 present under consideration do not seem to fall into any such 

 recognized combination, as evidenced not only by their peculiar 

 contour, but also by the peculiar superficial structure observed in 

 the typical representatives of the genus, and which is not found in 

 connection with the teeth of other genera occurring in the same 

 geological horizons. Hence, notwithstanding only the more conspic- 

 uous elements of the dentition of this genus are, at present, known, 

 these are marked by peculiarities which seem to warrant the recog- 

 nition of their distinct generic standing. 



While we have not the data necessary for carrying the compari- 

 sons to a final conclusion, we are much impressed by the inti- 

 mate resemblances which subsist between the present genus and 

 Tomodus, Agass. A series of careful sketches of authentic examples 

 of Tomodus convexm, Agass., from the Mountain limestone of Bristol, 

 England, and which were kindly submitted to us by Lord ENNISKILLEN, 

 show this relationship in a striking manner. The genus Tomodus, 

 indeed, appears to be a Xystrodus in all save the arrangement of 

 the punctae in parallel transverse lines; and in the latter respect 

 we find a close, though not absolute agreement, in the initial species 

 occurring in the American Carboniferous species X. simplex. 



Thus far ascertained, the genus Xystrodus is met with in both the 

 great divisions of the Carboniferous period. One of the writers, in 

 1877, described in Dr. HAYDEN'S final report on the U. S. geological 

 survey of Nebraska (p. 241, PL IV, f. 18), a tooth under the name 

 Xystrodus? occidentalis. The examination of subsequently acquired 

 and more complete suites of the fish- remains of our Coal-Measure 

 strata, from the upper division of which the latter specimen was 

 derived, has led us to relegate that particular specimen to the genus 

 Orthopleurodus. it probably being a very worn example, showing only 

 a part of the inner portion of a mandibular posterior tooth of the 



species 0. carbonarius, ( Deltodus angularis, N. and W.) 

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