324 PALEONTOLOGY OF ILLINOIS. 



is that by Prof. Mtjdge, of. a tooth represented PI. 8, Fig. 24, from the 

 Upper Coal Measures in the vicinity of Manhattan, Kansas. The latter 

 specimen pertained to one of the median rows, and exhibits in the 

 details of ornamentation and general conformation precisely similar 

 features possessed by the original specimen ; the only dissimilarity is in 

 the comparatively great elevation of the median cone. But we would 

 hardly venture to place great stress on this character alone — indeed, the 

 tooth from Danville presents an intermediate condition in this respect. 

 Hence, it would seem highly probable that we already possess authentic 

 evidence of the extensive stratigraphic range of this species. 



Gektjs PEKIPLECTEOD US, St. J. and W. 



Teeth symmetrical. Base more or less expanded laterally, or some- 

 times very compressed, symmetrically inrolled |from within outward, 

 sometimes embracing, as in the whorls of Natilus, again but gently 

 arched vertically, the lateral edges beveled and grooved, inferior surface 

 excavated. Crown consisting of a transverse series of strong median 

 cusps, flanked by at least a pair of small lateral denticles, one on either 

 side, and which regularly increase in size from the outer to the inner 

 extremity or with age: coronal cusps enameled, smooth or vertically 

 striated. 



The marked resemblance in form of the teeth above indicated to the 

 detached teeth of Plectrodus, Agassiz, of the British Upper Ludlow 

 "bone-bed," suggested the term by which they are here designated. 

 But their mode of implantation and apparent succession is quite differ- 

 ent from that of the Upper Ludlow teeth, the present teeth apparently, 

 as in the Cochliodonts, constituting a complete row, in which only the 

 coronal portion exhibits the individual elements and which are merged 

 into a common basal support. Taking the coronal region, a single set 

 of the transverse, cusps, the large median cone flanked on either side by 

 a small lateral denticle, and we have almost the counterpart of an indi- 

 vidual tooth of Cladodus. But here the intimate resemblance ceases; 

 the basal region scarcely presents the least element in common with 

 Cladodus, lacking the broad, posterior expansion — indeed the posterior 

 face of the base more inaikedly suggesting the anterior aspect of the 

 latter tooth. 



The symmetrical or exact vertical inrollment of the teeth would 

 appear to indicate that they occupied an isolated position, either con- 

 stituting the sole dental element of the fish by which they were pos- 

 sessed, or restricted to some part of the mouth other than the maxillary 

 elements; or, if associated with other and probably quite differently 



