VERTEBRATES. 



227 



occurring in the same deposits, it possesses characters of a much 

 more intimate nature in common with the same form of P. grandis 

 of the Keokuk limestone. But besides its much smaller size, the 

 rnandibular form departs widely from that of the Keokuk species, as 

 will be apparent on comparing the coronal contour of the two spe- 

 cies, as shown in the illustrations. The mandibular tooth bears 

 some resemblance to the maxillary form of P. angularis, but besides 

 its rugose ornamentation, the punctae exposed in the worn surfaces 

 are perceptibly coarser and more crowded than observed in that 

 species, which shows a uniformly minutely punctate surface, the 

 punctse being rather widely spaced. 



Geological position and locality: Chester limestone, Chester, Illinois. 



GENUS COPODUS, Agassiz. 



Copodus, AGASSIZ, MSS., 1859. (Psammodus cornutus, Agass., 1838, Poissons Fossiles, 

 tome III, p. 174; Catalogue of Type Specimens of Fossil Fishes in the Museum of the 

 Earl of Enniskillen at Florence Court, Ireland, Geol. Mag. VI, 1869; Dr. John J. 

 Bigsby, 1878; Thesaurus Devonico Carboniferus; etc. 



Teeth bilaterally symmetrical, spanning the jaw 

 without mesial suture, arranged in a single longi- 

 tudinal series from behind forwards. Lateral bor- 

 ders gradually converging anteriorly, where they 

 make an obtuse angle with the convex anterior 

 margin, posteriorly produced into an acute angle 

 meeting the lateral extremities of the concave pos- 

 terior margin. Coronal region slightly arched longi- 

 tudinally and laterally, distinctly denned from the 

 base. The lateral borders of the base form a more 

 or less prominent rim projecting beyond the coronal 

 limits, beveled or rounded inferiorly and more or 



GENUS COPODUS. , m1 



Maxillary form. less produced at the postero-lateral angles. Ine 



Fig. a. Triturating 



ST F?.' Transverse anterior and posterior walls vertical to the plane of 

 margin. from inrier the crown surface and channeled. Inferior surface 



Fig. c. Longitudi- , , , , ,-, 



nai profile. regularly arched transversely, and gently concave 



longitudinally in conformity with the arched coronal contour, the 

 tooth attaining maximum depth along the mesial line at the poste- 

 rior margin. The crown may present a lunate depression extending 

 a greater or less distance backward from the anterior margin and 



