286 PALAEONTOLOGY OF ILLINOIS. 



Htbocladodus plicatilis, St. J. and W. 



PI. V, Fig. 9. 



Teeth of medium or small size, symmetrical iu form. Base elliptical 

 in outline, gently arched vertically, lateral extremities neatly and 

 regularly rounded, the posterosuperior surface beveled to the broadly 

 expanded and rounded posterior margin, anterior face geutly arched, or 

 nearly straight, slightly recurved at the extremities, forming a well- 

 defined marginal ridge projecting downward and slightly outward, mod- 

 erately excavated below, and bordered posteriorly by a rather wide 

 beveled belt, sometimes coarsely striated or punctate. Crown occupy- 

 ing nearly the entire lateral diameter of the base, strongly constricted 

 basally in front, moderately so behind, lateral extremities obtuse, crest 

 sub acute, regularly rising into the prominent median cone, which is 

 lenticular in transverse section with delicate cutting edges, more or less 

 nearly vertical and slightly recurved ; the anterior face is marked by 

 rather strong vertical plicae which originate in slight protuberances, 

 sending off bifurcations above and below, those above producing a 

 delicate denticulation in the lateral portions of the crest, but which are 

 generally obsolete from abrasion, the plicae in the median cone percep- 

 tibly finer tban those in the lateral extremities ; the vertical plications 

 in the posterior face are very like those described, except that they may 

 be less strong. A medium-sized tooth is .30 inch in lateral diameter, 

 hight . 21, anteroposterior diameter of base . 12 inch. 



The individuals of this beautiful form vary considerably in size, 

 attaining twice the size of the specimen indicated in the above measure- 

 ments. However rare perfect teeth are, fragments of the crown are not 

 infrequent, and when these show any portion of the lateral extremities, 

 or the anterior median region of the base, they are not likely to be con- 

 founded with Cladodvs, which they somewhat resemble, as also Orodus, 

 especially worn specimens. 



Position and locality : Fish-bed of the Upper Burlington limestone ; 

 Louisa county, Iowa. 



Hybocladodtts tentjicostatus, St. J. and W. 

 pi. v, rig. io. 

 The collections from the Keokuk limestone afford a few imperfect 

 specimens of strongly cuspidate teeth, which are apparently referable 

 to the above genus, though specifically distinct from the last described 

 form from the Upper Burlington. None of the examples before us pre- 

 serve the base entire, but the symmetrical, gracefully taperiug median 



