326 PALEONTOLOGY OF ILLINOIS. 



accident one set of cusps has been displaced or crowded to one side, 

 thus destroying the perfect symmetry observable in^the normal con- 

 dition, and sometimes two median cones are placed side by side or in 

 alternating order. In certain examples of medium-sized teeth, which 

 show only three or four sets of coronal cusps, the anteroinferior margin 

 presents a broad, inbeveled area, in shape nearly corresponding to the 

 posterior face, and from its smooth condition appearing never to have 

 been articulated or cemented to the teeth in advance, which must have 

 preceded these more mature developments. The latter examples would 

 seem to indicate that, by accident, perhaps, the dental plate sometimes 

 presents two or more segments quite perfect in themselves, but parts 

 of the complete series as represented by a perfect specimen. Again, 

 in very young examples, showing three or four sets of cusps, we find the 

 same proportions as are maintained in the older examples. But there 

 occur in the collections very small specimens, showing as many as eight 

 set of coronal cusps, completely 7 inrolled or embracing, but with the 

 cusps and lateral basal borders so worn down as to leave little room to 

 doubt but that these minute examples are but part of the dental plate 

 which has become detached from the newer or posterior portion of the 

 series. This would indicate that a perfect, mature example may present 

 all the way from eight to twelve, or even more, sets of cusps, but those 

 of the anterior extremity being hidden from view by the inrollment of 

 the older extremity of the plate. 



The present form is dedicated to the memory of an esteemed young- 

 friend and companion, Master Wabeen Speingeb. 



Position and locality : Upper Burlington limestone ; Buffington creek, 

 Burlington and Augusta, Iowa. 



Pebiplecteodtjs compbessi t s, St. J. and W. 



PI. 8, Fig. 26 a, b, c. 



The present form is represented by a unique and perfect specimen, 

 discovered by Mr. Van Hobne. It is characterized by the extreme 

 lateral compression of the base, the lateral walls of which are but 

 slightly produced outward in their desceut and relatively shallow, 

 defining the lateral boundaries of the correspondingly narrow, deeply 

 channeled inferior surface, and but moderately arched antero-posteriorly. 

 The crown partakes equally in the lateral compression of the basal 

 region, the specimen showing four sets of cusps, of which the two ante- 

 rior ones are much worn by attrition; the median cones are relatively 

 very large, the long diameter of their elliptical transverse section 

 extending in the direction of growth or antero posteriorly, instead of 

 transversely as in the preceding form ; they taper to the rounded apex, 



