VERTEBRATES. 175 



GENUS XYSTRODUS, Agassiz. 



XYSTRODUS, Agassiz, MSB., 1859 (COCHLIODUS STRIATUS, Ag. Poios. 

 Foss. Ill, p. 174; XYSTRODUS ANGUSTUS, Ag. Mas., 1859. 



The genus Xystrodus was recognized by Professor Agassiz in 1859, 

 to which he transferred the species previously described under the 

 name Cochliodus striatus, Ag., from the Carboniferous limestone of 

 Ireland. The later term was accepted by European palaeichthyolo- 

 gists, and at a later date, 1869, was published in the list of type 

 specimens of the magnificent collection at Florence Court, belonging 

 to Lord Eiiniskillen, as also in the lists of the British Museum and 

 and other institutions, and incorporated in the great work by Dr. 

 Bigsby, the "Thesaurus Devonico-Carboniferous." 



In accordance with the presumptive determinations and nomencla- 

 ture adopted in the present work, the original or type of the genus 

 Xystrodus striatus is referable to the maxillary posterior form, and not- 

 withstanding the opposed mandibular teeth have been obtained from 

 the same horizons and localities affording the former, we are not 

 aware of their having been specifically identified and described. 

 The latter teeth are recognizable by the extreme alation of the 

 postero-lateral border, giving to the general figure excessive breadth 

 compared to the length in the direction of inrollment. 



The above-mentioned teeth agree in all essential features with the 

 forms occurring in the various American Lower Carboniferous forma- 

 tions. Especially is this notable in the case of the Chester teeth 

 Xystrodus verus, which indeed constitutes a typical' representative Of 

 the genus. Therefore, from such materials as we possess, the fol- 

 lowing diagnoses of the opposed terminal teeth of the upper and 

 lower jaws are prepared; of the other dental forms with which they 

 were associated during the life of the fish to which they belonged, 

 as yet we have failed to arrive at even a surmise as to their char- 

 acter. 



Mandibular posterior teeth distinguished by their triangular out- 

 line and great transverse breadth of the inner margin as compared 

 with the longitudinal diameter. Coronal surface occupying the entire 

 superior area of the tooth, comparatively plain, more or less con- 

 cave, or with a longitudinal depression, the coronal prominence 

 crowded to the anterior side, nearly flat or but gently convex trans- 

 versely, at the opposite border terminating in the expanded, upraised 



