362 PALAEONTOLOGY OF ILLINOIS. 



must have been very hard. Though obscurely granular throughout, ifc can 

 hardly be said to be anywhere punctate. 



Formation and 'locality : Coal Measures; La Salle, 111. 



GENUS PELTODUS, N. and W. 



TEETH small and low, round, oval or elliptical in outline, 

 arched above in both directions, concave or flattened below ; 

 crown surface most strongly arched from front to rear, highest 

 near the anterior margin, more or less evenly punctate through- 

 out; under surface bony and rough; margins thin and irregu- 

 lar where the teeth are separated, thickened and even along 

 the lines of contact when closely set. 



These teeth indicate a dentition intermediate in character between that of 

 Psammodus and Cochliodus ; less flat, smooth and pavement-like than the for- 

 mer ; less convoluted than the latter ; though, doubtless, performing the same 

 duty crushing the shells of mollusks for which the teeth of so many of the 

 Cestracionts were employed. They are smaller and thinner than the teeth of 

 the genera which have been cited, Sandalodus, Deltodus, etc., and apparently 

 belonged to the humbler members of the great group of Selachians which in- 

 habited the Palaeozoic sea. The type species, P. unguiformis, is found in the 

 calcareous beds of the Coal Measures, and forms a distinct and interesti ng ele- 

 ment in the small Cestraciont fauna of that epoch ; the few and feeble repre- 

 sentatives of the shoals of sea monsters which lived in the epoch that immedi- 

 ately preceded. 



Among the fish teeth from the Lower Carb. strata contained in the collection 

 there are some which so much resemble these in general character, that they 

 have been placed in the same generic group under the name of P. pulvinulus- 

 They are longer, thicker, more elongated laterally, and much more coarsely punc- 

 tate. 



In P. unguifcrmis, the broader, anterior end bears marks of attrition, and it 

 is evident that they were so placed on the jaw that the anterior margin was 

 most elevated and took all the wear to which the tooth was subjected. From 

 this it will be seen that, though resembling some of the Petalodont teeth in gen- 

 eral form, they were placed on the jaw in reversed position from them, as 

 though the teeth of Antliodus were turned with the concavity down. The dis- 

 tinction of crown and root, and the imbricated enameled folds visible in all the 

 Petalodonts are in Peltodus entirely wanting. 



