134 PALEONTOLOGY OF ILLINOIS. 



the " extreme angles of the inner margin 13 mm. ; length along 

 antero-lateral border 3 mm. 



The above described forms are about equally numerously repre- 

 sented in individuals in the collections, although the specimens are 

 in a more or less fragmentary state of preservation. The discovery 

 by Mr. Van Home of the mandible bearing the posterior pair of 

 teeth, and showing the proximal articular extremity the right 

 ramus and the somewhat produced, laterally narrowed extremity in 

 the region of the symphysis, is the most interesting in connection 

 with the genus. The semi-ossified tissue of the mandible in this 

 instance seems to have remarkably intimate connection with the 

 dental plates, and preserved in such a manner as to have the 

 appearance of a cylindrical scroll, the inferior or inner margin 

 being brought round to the exterior side, as the specimen rests in 

 the limestone matrix. It forms a thin plate whose walls have been 

 brought into contact by pressure, but posteriorly it probably 

 shows its normal shape and dimensions, where it presents a cres- 

 centiform excavation with the condyloid or inferior process most 

 produced posteriorly, the superior process relatively strongest. An- 

 teriorly in the region of the symphysis, the mandible is less dis- 

 tinctly outlined. It is moderately produced and apparently obtusely 

 rounded, the rami meeting at the symphysis but not anchylosed, 

 and bearing either side a slight protuberance elongated parallel with 

 the symphysical line, but without a vestige of the dental plates to 

 which they afforded support. The latter more evidently of very 

 small size, and if, as in Cochliodus, the distal extremity of the rami 

 were armed with a median and anterior convoluted plate, their ex- 

 treme diminutiveness may account for their absence in the collec- 

 tions. The same conclusion is also permissible in reference to the 

 anterior dentary elements of the upper jaw. 



Geological position and localities: St. Louis limestone ; St. Louis, 

 Mo. ; Alton, 111. ; Pella, Iowa. 



