310 PALEONTOLOGY OF ILLINOIS. 



ter, and but slightly produced posteriorly to the vertical plane of the 

 crown, subrectangular iii outline as seeu from the posterior face, which 

 latter is gently arched laterally, nearly plane vertically and faintly 

 beveled below, anterior face rather deeply excavated, shoulder well- 

 defined, produced into a delicate basal protuberance beneath the coro- 

 nal angulation, inferior surface well defined and very obliquely beveled 

 to the thin posterior edge; surfaces coarsely pitted. A specimen of 

 medium size measures in lateral diameter .17 inch, antero-posterior 

 diameter .06 ; the abbreviated variety is proportionately shorter aud 

 wider in the same diameter, and the base relatively deeper. 



The above species, examples of which were first discovered by Mr. 

 Van Horne, is numerously represented in our collections, the majority 

 of the specimens being in a very perfect state of preservation. It is 

 usual to find three teeth arranged in a row, and some specimens present 

 as many as seven teeth thus associated, and which are sometimes appa- 

 rently firmly co-ossified at the impingement of their bases. That the 

 consolidation of the teeth upon a common basal support is not the nor. 

 mal condition, is shown not only by the examples, which are visibly 

 separated by suture, but also by those detached individuals in which 

 the basal portion presents no indication of such intimate union with the 

 teeth of the same vertical series. Both the short and the elongated 

 varieties occur in the above condition. Iu examples in which several 

 teeth are associated, the same general proportion of parts is preserved 

 in each individual of the series, except that in the small terminal teeth 

 the crown is more or less abraded, other individuals exhibit eccentric 

 enlargement of one or other extremity, and along the inner border of 

 the series is sometimes present the coronal cap of an immature newly 

 formed tooth ; in the number of tuberculations of the coronal crests 

 there is slight variation. 



These teeth constitute a remarkably well-defined specific form, the 

 variations of which are more dependent on individual proportions than 

 on the modification of the characters which equally distinguish the 

 abbreviated robust as well as the elongated teeth, as intimated in the 

 foregoing description. Intimately allied to D. minuscutus of the Keo- 

 kuk limestone, it is distinguishable from that form by the strongly 

 defined coronal fold, the elongated teeth by their broader and more 

 regularly elliptical outline, while the short teeth differ from the corres- 

 ponding variety of the above form in the proportionately greater eleva- 

 tion of the posterior crown face and the more strongly arched basal line- 



Position and locality : Although the teeth are found in greater or less 

 abundance in certain layers, they range through a vertical thickness of 

 strata of thirty feet, more or less, in the upper part of the St. Louis 

 limestone ; Alton, Illinois, St. Louis, Missouri. 



