VERTEBRATES, 37 



Petalodus linguifer, N. and W. 



PI. II. Figs. 4, 4 a, 4 b, 5, 5 a, 5 4, 5 c. 



Teeth large, broader than high ; crown three times as broad 

 as high, straight or very slightly arched laterally, cutting edge 

 broadly and nearly evenly arched, apiculate at the summit, 

 strongly striated before, less so behind, finely crenulated; ante- 

 rior surface very broadly rhomboidal, highly polished; basal 

 imbricated folds narrow; posterior face slightly higher than 

 anterior, like that broad, rhomboidal in outline, smooth and 

 highly polished, much arched vertically, very slightly so later- 

 ally, bordered below by a broad band of 7-8 imbricating folds 

 of enamel; root smooth except where peirced by the nutrient 

 vessels, accurately tongue-shaped, rounded below, posterior face 

 equal in height to the posterior face of the crown, including 

 basal folds; anterior face one-third the higher. Total height 

 of tooth, 12 lines; breadth, 18 lines. 



In the form of the crown, these teeth hear considerable resemblance to those 

 of P. acuminatus, Ag\, but differ in having the upper lines of their rhombic 

 outline convex instead of concave ; the root, too, is less wide. In P. acumina- 

 tus — as would appear from the figures of Agassiz and McCoy — it is as broad 

 where it unites with the crown, as the crown itself; while in P. linguifer it is 

 much narrower, and is precisely of the form of the terminal half of the human 

 tongue. In the very beautiful and perfect specimen from Pope county, (Fig. 

 4), the tongue-like I'oot is broader than in those from Chester, (Figs. 5, 5c), 

 but is still much narrower than in the described specimens of P. acuminatus. 



It is, perhaps, possible that the Chester specimen should constitute a distinct 

 species, but with the exception of the difference in the form of the root, they 

 seem to be alike. 



It will be seen from the sections given (Figs. 5 6, 5 c) of two specimens 

 from the Chester limestone, that they show a marked difference in the relative 

 height of the crown, but this is a character which is particularly inconstant in 

 all the species of Petalodus. 



Figures 4 and 4 a, a perfect specimen from Pope county, seen from behind 

 and above; 4 b, section of same; 5, 5 a and 5 b, anterior and posterior faces, 

 and section of a worn specimen from Chester ; 5 c, profile section of another 

 Chester specimen. 



Formation and locality: Chester limestone, Chester and Pope county, Illinois. 



