40S PALAEONTOLOGY OF ILLINOIS. 



Position and locality : Upper beds of the St. Louis limestone ; Pella, 

 Iowa; Alton, Illinois, and St. Louis, Missouri. 



Petalorhynchtjs spatulatus, St. J. and W. 



PI. XXI, Fig. 5,0. 



Associated with the preceding species occurs a form of teeth which 

 present peculiarities strikingly in contrast with the distinguishing fea* 

 tures of P. pseudosagittatus, and holdiug the same relation to that form 

 in the St. Louis limestone, as does P. psittacinus with P. sagittatus in 

 the Carboniferous limestone of Armagh. The chief distinguishing 

 peculiarity consists in the swollen condition of the basal portion of the 

 teeth, which is particularly noticeable from the convex side, the base 

 presenting a massive, laterally expanded surface, elliptical in outline, 

 very convex laterally and compressed in the gently rounded lateral 

 margins, broadly rounded below, slightly converging above and com- 

 pressed beneath the lateral portions of the coronal margin ; opposite 

 face less than half the vertical extent of that described, laterally flat- 

 tened, perhaps faintly depressed, strongly rounded to the extremity, 

 both faces more or less roughened, with irregularly disposed foramina. 

 Crown proportionately strong, convex face broadly arched laterally, 

 lateral margins sharp, more or less rapidly converging towards the 

 strongly acuminate apex, which is sometimes truncated or rounded, 

 basal margin slightly prominent, broadly arched downward in the 

 median region aud more or less strongly curved laterally, the rather 

 wide, gently beveled inferior belt occupied by three or more regular 

 imbricating folds, which gradually narrow towards the extremities, aud 

 apparently continuous with the coronal belt of the opposite face ; con- 

 cave crown-face twice the Light of the convex face, subovate in outline, 

 moderately excavated, basal margin well-defined from the base, strongly 

 arched downward in the middle and lightly produced at the lateral 

 extremities, coronal belt wide and composed of several imbrications — 

 in the specimen before us the folds are very obscure as though from 

 abrasion. Vertical dimensions of a tooth of large size .90 inch, greatest 

 lateral diameter of base .45, elevation of convex crown-face .38 inch. 



The collection of Mr. Van Hoenb contains three examples of the 

 present form, of large and medium size, none of which, however, show 

 the concave aspect of the teeth. But a fragment of the basal portion 

 of one of the specimens plainly reveals the outline of the concave 

 crown-face, with the strongly downward arched basal belt, which is 

 apparently made up of several imbricating folds, the upper folds being 

 slightly arched upward in the middle, much in the same manner as 

 observed in P. pseudosagittatus, though relatively narrower than in the 



