80 PALAEONTOLOGY OF ILLINOIS. 



Geological position and locality: Chester limestone, Chester, Illi- 

 nois. 



GENUS YATICINODUS 1 , St. J. and W. 



The distribution and investigation of the materials illustrating 

 Deltoptycldus and allied genera, have brought into prominence homol- 

 ogous forms of teeth which range themselves under a group appar- 

 ently characterized by persistent features, in contradistinction to 

 Deltoptychius on the one hand, and Stenopterochis on the other, 

 and which, if we are correct in inferring their generic distinctness, 

 offer a premonition of the later differentiated Orthopleurodus, etc. 

 Unfortunately, however, the state of preservation of the material 

 illustrating these forms is in no instance such as leads to indubita- 

 ble conclusion in regard to their generic distinctness from Stenop- 

 terodus. Hence the various specific categories into which the forms 

 from the several formations readily resolve themselves, are pro- 

 visionally recognized under th'e above generic designations. 



The forms above referred to represent the posterior teeth of the 

 upper and lower jaws. The maxilliary form is in every respect like 

 the corresponding teeth of Deltoptychius, save in the apparent 

 absence of the least vestige of the presence of the secondary lobe, 

 the entire anterior portion of the tooth forward of the posterior 

 prominence presenting a plain surface, such as obtains in Stenop- 

 terodus. The anterior-lateral border is probably truncated from the 

 inner angle slightly obliquely outward and forward, in which respect 

 it differs from the prevalent condition observed in typical Deltop- 

 tychius and Stenopterodus. The mandibular form is also in general 

 outline and contour like that of the former genus ; but the indi- 

 viduals here referred to do not preserve a trace of the narrow plain 

 belt such as in Deltoptychius, distinctly defines the median lobe from 

 the edge of the abrupt antero-Jateral border. In the latter particular 

 the teeth referred to bear striking resemblance to the mandibular 

 posterior form of Orthopleurodus. As all of the few examples of 

 this form are represented by evidently worn individuals, it cannot 

 be denied that they may be merely abraded teeth of Deltoptychius. 



With one or two exceptions, the species noticed in this connection 

 are unquestionably distinct from those hereinafter described under 

 the head of Deltoptychius. These possible exceptions relate to the 



l Vaticinus, prophetical; odous, tooth. In allusion to its being the supposed earliest of 

 the Cochliodonts. 



