224 PALAEONTOLOGY OF ILLINOIS. 



the upper jaw of the above species is recognized in the teeth which 

 Messrs. Ne wherry and Worthen identified with Psammodus porosus 

 of Agassiz. These teeth are evidently most intimately related, so 

 far as may be judged from the similarity in the details of their su- 

 perficial coronal structure, which is, indeed, identical in both of the 

 forms. We are, therefore, convinced that they represent forms of 

 one and the same species, presenting the same combination of den- 

 tal apparatus observed in the several species described in the fore- 

 going pages. 



The latter form is distinguished by its transversely elongate tra- 

 pezoidal outline, the course of the lateral borders, and general eon- 

 tour of the coronal region intimately repeating those parts as observed 

 in Psammodus plenus, of the St. Louis foimation. The inner border, 

 which is similarly marked by a slight angulation a little in advance 

 of the middle, shows a distinct truncation of the posterior angle 

 with a narrow articular facet. In the character of the anterior and 

 posterior margins, also, intimate resemblances are noticeable with the 

 above species. But the examples of the present form are apprecia- 

 bly of greater depth and more solid or massive proportions, and the 

 punctffi of the coronal surface are also perceptibly firmer and more 

 numerous even than remarked in the corresponding teeth of P. 

 plenus. A mature tooth measures in greatest transverse diameter 

 across the inner margin 39 mm. ; length along inner border 23 mm. ; 

 greatest depth of tooth 10 mm. 



Together with the associate form, both forms occurring in the 

 same horizon, the species is readily distinguished from that with 

 which it has already been compared, and of which it is an interest- 

 ing example of a representative species. As regards their relations 

 with the European teeth originally described by Professor Agassiz under 

 the name of Psammodus porosus, we deem it hardly probable that 

 that species is so variable as to include the present forms. 



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

 PSAMMODUS RETICULATUS, N. and W. 



PL XIX, Fig. 3, 5. 



Psammodus reticulatus, NEWBEBKY and WOKTHEN, 1866, 111. Geo. Sur., II, p. 109, PL XI, 

 f . 5, 5a. 



Besides the type specimen, the collections from the same horizon 

 whence that was derived contain other imperfect examples, appar- 

 ently representing the two forms ascribed to the upper and the lower 

 jaws, which are believed to be specifically identical with the tooth 



