180 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



veloped into a perforation passing entirely through the horizontal portion of 

 the crown, a condition which is sometimes observed in P. falcatiis. The York- 

 shire species known as P. benniei (Etheridge) differs from both P. falcatus and 

 P. semicircxdaris in that the coronal margin of the upper tooth is not dentated 

 but smooth, and rises into an acuminate apex in front. 



The original of Plate 2, Fig. 5, possesses some pathologic interest, inasmuch 

 as it became deformed during life, either as the result of injury or of irregu- 

 larity in growth. It is an upper tooth shown here in left lateral aspect, and 

 both root and ci'own on the side away from the observer are strongly indented. 

 Fine parallel scratches resulting from the attrition of food, and preserved as dis- 

 tinctly as in a fresh individual, extend in the same direction over both the 

 inner and outer coronal face on the uninjured side, and their obliquity to the 

 vertical axis indicates that the tooth stood slantwise in the jaw, only about half 

 the cutting-margin functioning against the lower tooth. Had its position been 

 erect in the jaw, these markings would of course have been vertical, as in all 

 normally formed teeth. The serrations of the cutting-margin have become 

 almost effaced through wear. The triangular form of the root (as seen in 

 profile) is natural, and the sublunate surface for its attachment to the crown 

 is well shown in the original of Plate 2, Fig. 7. The latter tooth is de- 

 tachable from the matrix, thus exposing a mold of the posterior face. Im- 

 pressions in the matrix show that the cutting-margin was prominently 

 serrated, as in the original of Fig. 6 of the same plate, the root of which has 

 not been freed from the matrix. For comparison with the Nebraska speci- 

 mens shown in Plate 2, an illustration is given in Plate 3, Fig. 25, and also in 

 Text-figure 8, of a large upper tooth from the summit of the Chester limestone 

 in Kentucky. The originals of this and also of the lower tooth shown in Text- 

 figure 7 were found by Mr. E. O. Ulrich in such close proximity at the same 

 outcrop near Montgomery Switch, Caldwell County, as to leave scarcely any 

 doubt that they pertained to a single individual. 



Formation and Locality. — Atchison shales (Missourian) ; Bellevue, Ne- 

 braska City and South Bend, Nebraska. Productive Coal Measures ; Ohio 

 and Indiana. Chester Group ; Caldwell County, Kentucky. 



COCHLIODONTIDAE. 



PLATYXYSTRODUS Hay. 



The name Platyxystrodus has been proposed by 0. P. Hay as a substitute 

 for the preoccupied title of Xystrodus, the latter having been employed by 

 Plieninger two years prior to the application of the term in 1860 by Morris 

 and Roberts. 



