1883.] NATURAL SCIENCES OF PHILADELPHIA. 155 



The second tooth is broadly molar. Two foramina perhaps indi- 

 cate the position of two teeth of an internal row. The toothless 

 apex of the bone is longer and flatter than in 31. robustus. The 

 entire bone is flatter than in that species. The first tooth stands 

 on the edge of the symph3^sis. 



Measurements. m. 



Total length on tooth row, '025 



Length of base of tooth row, . . . . 'OlS 



Length of apex, ....... '016 



Width at middle, -018 



Near Sinker's Creek, Idaho. J. L. Wortman. 



Mylooyprinns robustus Leidy. Loc. cit. Report U. S. Geol. Survey Terrs., i, p 

 262, PI. XVII, figs, 11-17. 



This is the most abundant fish of the Idaho beds, and is repre- 

 sented by a great many pharyngeal bones with teeth, in my col- 

 lection. These present a great many variations, and I have 

 proposed in a former paper to recognize three species: 31. kingi, 

 31. robustus and 31. longidens. Study of m}^ material shows that 

 these forms intergrade, and that if they represent distinct species, 

 two others must be admitted. I incline to look upon the differ- 

 ences as due in part to age, and in part as subspecific variations. 

 I tabulate them as follows : 



1. Small; style more slender, five teeth in outer row, the 

 upper very small and subprehensile ; the lower small, 

 conic. 

 II. Like the last, but the style stouter. 

 III. Like I, but only four teeth ; the inferior tooth wanting. 

 . IV. Like I, but four teeth; the superior larger and obtuse; 



31. longidens. 



T. Larger ; four teeth, the last obtuse but much smaller than 



the others ; style stout; 31. robustus. 



YI. Larger; style stout; four teeth, the superior nearly as 



> large as the others, which are equal ; 3f. kingi. 



The slenderness or stoutness of style is not coincident with the 



other characters, but the latter condition is always found in large 



specimens. In these the convex border is also much thickened. 



The small, partly hooked form of the superior tooth is only found 



in small fishes, and is probably a character of j^outh. It indicates 



that the genus is descended from more purely carnivorous types. 



