32 PALAEONTOLOGY OF ILLINOIS. 



The difficulties which stand in the way of any attempt to establish generic 

 and specific distinctions, among extinct plagiostomous fishes, from the teeth as 

 they are usually found, detached from their connections and scattered through 

 the rock containing them, are strikingly illustrated by the group of Petalodoid 

 teeth before us. 



This collection embraces nearly one hundred specimens, in various states of 

 preservation — many of them beautifully perfect — which exhibit modifications 

 of a common type of form, and, so far as they have been been examined micros- 

 copically, a common structure ; and yet it is quite impossible to bring them all 

 within the limits of any one of the allied genera established by Owen, Agassiz 

 or McCoy. Selections may be made from them distinctly representing Petalodus, 

 Polyrliizodus, Chomatodus, etc.; but with these are intermediate forms, which 

 serve as connected links between the others, and make it difficult, if not impos- 

 sible, to break the series up into satisfactory and well defined generic groups. 

 Doubtless if we could have for study the entire organism now so dimly shadowed 

 forth in these numerous, variable and disconnected teeth, we should find in the 

 form, in the complete dentition or in other organs — the fins, the spines, the 

 bronchial openings, etc. — characters by which the group of fishes here repre- 

 sented might be arranged in a large number of well defined genera, the commu- 

 nity of form and structure which their teeth exhibit would probably then be. 

 shown to be a family, and not a generic character. 



P. Hastingsi, Owen (type). Sub-carboniferous, Europe. 



P. acuminatus, Ag. Sub-carboniferous, Europe. 



P. destructor, N. and W. Coal Measures, Illinois. 



P. linguifer, N. and W. Sub-carboniferous, Illinois. 



P. alleghaniensis, Leidy. Coal Measures, Penn. and Ohio. 



2. Genus PETALORYNCHUS, Ag. 



Teeth small; crown compressed, thin, concavo-convex petal 

 shaped, relatively higher and narrower than in Petalodus; im- 

 bricating folds on posterior face forming a short, transverse 

 band, not extending to the lateral angles of the crown; root long, 

 undivided. 



S. sagittatns, Ag. — name only — (type). Sub-carb., Europe. 



P. psittacimis, Ag. — name only. Sub-carboniferous, Europe. 



P. striatus, N. and W. Sub-carboniferous, Illinois. 



